July 13, 2026 • 14 mins
All colors only really exist in the mind of the beholder -- and there are colors that we can't perceive, or can only perceive under particular circumstances. Learn about impossible colors and how to see them in today's episode of BrainStuff, based on this article: https://science.howstuffworks.com/impossible-colors.htm
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Speaker 1 (00:01):
Welcome to Brainstuff, a production of iHeartRadio, Hey, brain Stuff,
Laura vogelbaumb here. Even at full function, our eyes don't
provide us with a complete picture of the world around us.
There are plenty of things we can't see under normal circumstances,
like ultraviolet wavelengths or impossible colors like stiggy and blue.
(00:25):
Sticky and blue if you're unfamiliar, is what's called a
chimerical color. It's an after image. Specifically, it's the color
you perceive when you stare at a bright yellow circle,
say for several seconds, and then look at a black square,
you'll see an after image of a deep blue circle,
a blue that's just as dark as the black against
(00:47):
the black square. The color is named after the underworld's
river sticks. Of course, a blue that's as dark as
black is impossible, but our weird brains nevertheless perceive it
if it helps. There's actually no such thing as blue
to begin with, or red or green or fusia. These
(01:09):
are just words we have for particular sensory experiences. Color
exists purely in our minds. A banana, for example, is
not inherently yellow. To prove it is, stumble to your
kitchen in the middle of the night and hold a
banana in front of your face. What color is it then?
(01:29):
Kind of a muddy grayish black, probably definitely not bright yellow.
And that's because colors are not emitted from most objects.
They are reflected. A banana is yellow because when visible
light hits it, the wavelengths that we call yellow are
what bounce back. We've talked about light and color before.
(01:50):
A white light from the sun or a normal light
bulb is composed of wavelengths spanning the entire visible spectrum.
You know your basic ROYGBIV, going from the longest wavelengths
to the shortest. That's red, orange, yellow, green, blue, indigo,
and violet. But when white light shines on a banana peel,
(02:11):
something frankly incredible happens. There's a natural pigment in the
peel called xanthophyl that's chemically programmed to absorb certain wavelengths
of light and reflect others. The dominant wavelength reflected by
xanthophyl is yellow, but the yellow of that banana still
doesn't exist as such. It only starts to exist when
(02:33):
the reflected light from that peel is detected by millions
of color sensing cells in your retinas called cones. Most
humans have three types of cone cells, referred to as red, green,
and blue cones, or more accurately as long, medium, and
short cones, because each are more sensitive to wavelengths of
(02:53):
light in those ranges. The cones send a multitude of
electrical impulses to the brain that data is processed. Each
cone can process one hundred or more different shades of color,
giving your average human a range of a million to
ten million perceivable colors. Some evolutionary biologists think that this
(03:15):
trichromatic color vision evolved in primates to help us spot
colorful berries amongst foliage. But we know that other animals
perceive color differently because they evolved to have different numbers
or types of cones. Dogs, for example, have just two
types of cones, blue and yellow. They don't really perceive
(03:36):
reds and greens. Most birds have four types of cones, red, green, blue,
and ultraviolet, meaning that they can see colors we can't.
Honeybees can see into the infrared part of the spectrum.
Now ultraviolet and infrared light exist, we know they do.
We can observe them in other ways, and we know
(03:58):
that what we call yellow light exists. It's the part
of the electromagnetic spectrum with wavelengths around five hundred and
seventy two five hundred and eighty nanometers or thereabouts. But
other colors that we perceive technically don't exist, like sticky
and blue, or think about magenta. Magenta is what we
(04:21):
see when our brains process a combination of red and
blue light. But red and blue are on opposite ends
of the visible spectrum. There is no wavelength for magenta.
On the flip side of the coin, people with red
green color deficiency might not be able to cite the
difference between red and green apples. For them and for
(04:43):
the best dog in your life, those colors don't exist.
The moral of this color story is this, without our
visual system and without our brains, no color really exists.
It's all in the mind of the ball, which is
enough to make you say dude, And it leads to
(05:06):
a fascinating question. Are there colors within the visible spectrum
that our cones and brains can't see? The so called
impossible colors or forbidden colors break the biological rules of perception.
We're going to get more into impossible colors, but first
(05:27):
we're going to get into a quick break for a
word from our sponsors, and we're back. Thank you sponsors. Okay,
to talk about this, let's start by digging deeper into
the science of color perception. Each of your eyes contains
roughly six million cones concentrated in the center of the retina.
(05:50):
These cones come in those three different varieties types that
pick up on short, medium, or long wavelengths of visible light.
When a cone receives a strong sign in its wavelength zone,
it sends electrical impulses to the brain. The brain's jobs
to combine the millions of electrical signals from each cone
to recreate a composite image of the color that you're
(06:12):
looking at. The brain, of course, is not a computer,
but rather has a complex lump of highly specialized cells
called visual cortex that's responsible for processing the electrical signals
from the cones. We're still learning how these cells work
and work together and with other regions of the brain
(06:33):
to process input from the eyes. But basically the color
related cells are called opponent neurons, and they traditionally have
been separated into two basic types, red green opponent neurons
and blue yellow opponent neurons. They're called opponents because they
function in a binary way. The red green opponent neuron
(06:55):
can signal either green or red, but not both simultaneou,
and the blue yellow opponent neuron can signal either blue
or yellow. I feel obligated to say here that recent
research has indicated that this explanation is actually a culturally
convenient oversimplification of what seems to be a three ish
(07:18):
step brain process that more precisely involves lavender lime opponent
neurons and salmon teele opponent neurons of five varying opponentcies
that can combine to red versus green, and blue versus yellow.
But since I'm not a neuroscientist, oh, let's stick with
the convenient oversimplification for today. Okay, So, opponent neurons how
(07:44):
they basically work is this. When you look at a
pure yellow image, the yellow portion of the blue yellow
opponent neuron is excited and the blue portion is inhibited.
Switch to a pure blue image, and the blue portion
of the opponent neuron is excited while the yellow is inhibited.
And now imagine trying to see an image that's equally
(08:06):
blue and yellow at the same time. The opponent neurons
can't be both excited and inhibited simultaneously. That's why bluish
yellow is an impossible color. The same is true for
reddish green. You might be saying, I know what yellow
and blue look like together. It's green and red and
(08:27):
green make a kind of muddy brown. But that's the
result of mixing two colors together, not of a single
pigment that's equally blue yellow or red green. All the
way back in eighteen oh one, long before scientists knew
about cones and neurons, English physician Thomas Young theorized that
the human eye has three types of color receptors, blue, green,
(08:51):
and red. Young's trichromatic color theory was proven correctsh in
the nineteen sixties, when cone cells, named for their shape,
were to discovered to have special sensitivity to about those wavelengths.
The opponent color theory of perception has been around since
the eighteen seventies, when German physiologist Edvald Herring first postulated
(09:12):
that our vision was ruled by opponent colors across from
each other on the color wheel, with the corset being
red versus green and blue versus yellow. This theory is
simple and intuitive enough that it's still repeated in physiology textbooks,
but in the past five to ten years, neural imaging
has shown that what's going on in our brains is
(09:33):
more complicated. Still, it's functional enough to roll with for today.
So taken together, trichromatic theory and opponent theory argue that
it is impossible for the human eye and mind to
perceive certain colors described as red, green, or blue yellow. However,
(09:55):
as Luigi wisely told us in the nineteen ninety three
Super Mario Brothers Life live action film, nothing's impossible, improbable, unlikely,
but never impossible. In that spirit, in the early nineteen eighties,
visual scientists hewet Crane and Thomas Piantinada designed an experiment
(10:15):
with the goal of tricking the brain into seeing impossible colors.
In their experiment, subjects were instructed to stare at an
image of a vertical red stripe adjacent to a vertical
green stripe. The subject's heads were stabilized with a chin rest,
and their eye movements were tracked by a camera. With
every tiny twitch of a subject's eyes, the redd and
(10:37):
green image was automatically adjusted so that the subject's gaze
remained fixed on the opposing colors. It turned out that
if people stared at adjacent opposing colors for long enough,
the border between them would dissolve and new impossible colors
would emerge. The resulting colors were so new that subjects
(10:59):
had a hard time describing them. By stabilizing the image
to track eye movements, the researchers theorized that different areas
of the eye were being continuously bathed in different wavelengths
of light, causing some opponent neurons to get excited and
others to be inhibited at the same time. But after
several other researchers failed to achieve the same dramatic results,
(11:23):
this experiment was dismissed as a parlor trick. Subjects, instead
of seeing brand new hues of greenish red or bluish yellow,
usually described the blended color as mud brown. Others would
see fields of green with pixelated red dots scattered across it.
Impossible colors became a scientific joke. But then in twenty ten,
(11:47):
biophysicists Vincent Bellick and Brian Zoe published results that they
believed explained why Crane and Piontinita had succeeded where others
had failed. They identified the combination of eye tracking and
luminance or brightness as being key to tricking the brain
into seeing impossible colors. They ran their own experiments in
(12:10):
which subjects were again strapped to a chin rest and
monitored by the latest retinal tracking technology, With the images
stabilized to these subject's eye movements. The researchers played with
the luminance of the two opposing color stripes. When there
was a difference in brightness, the subject's experienced to the
pixelated colors reported in earlier experiments. But when the two
(12:34):
colors were exactly the same brightness, then six out of
seven observers saw impossible colors. Even better, two of them
could see the new colors in their minds for hours
after the experiment was over. While few of us have
a retinal stabilizer in the basement, there are some simpler
exercises that can temporarily trick the brain into seeing the forbidden.
(13:00):
Direct is to stare at an image of two opposing
color squares side by side, each with a white plus
sign in the middle. You can find examples of this online.
What you do is relax and cross your eyes until
the two plus signs merge into one. I tried this
the blue, yellow one was weird, and the red green
(13:22):
one made me laughably uncomfortable. Sort of makes you wonder
what else is out there that we can't see yet.
Today's episode is based on the article how impossible colors
like Stiggy and Blue work on how stuffworks dot Com,
written by Dave Ruse. Brain Stuff is production of iHeartRadio
(13:43):
in partnership with how stuffworks dot Com and is produced
by Tyler Klang. Four more podcasts from my heart Radio.
Visit the iHeartRadio app, Apple Podcasts, or wherever you listen
to your favorite shows
Welcome to Brainstuff, a production of iHeartRadio, Hey, brain Stuff,
Laura vogelbaumb here. Even at full function, our eyes don't
provide us with a complete picture of the world around us.
There are plenty of things we can't see under normal circumstances,
like ultraviolet wavelengths or impossible colors like stiggy and blue.
(00:25):
Sticky and blue if you're unfamiliar, is what's called a
chimerical color. It's an after image. Specifically, it's the color
you perceive when you stare at a bright yellow circle,
say for several seconds, and then look at a black square,
you'll see an after image of a deep blue circle,
a blue that's just as dark as the black against
(00:47):
the black square. The color is named after the underworld's
river sticks. Of course, a blue that's as dark as
black is impossible, but our weird brains nevertheless perceive it
if it helps. There's actually no such thing as blue
to begin with, or red or green or fusia. These
(01:09):
are just words we have for particular sensory experiences. Color
exists purely in our minds. A banana, for example, is
not inherently yellow. To prove it is, stumble to your
kitchen in the middle of the night and hold a
banana in front of your face. What color is it then?
(01:29):
Kind of a muddy grayish black, probably definitely not bright yellow.
And that's because colors are not emitted from most objects.
They are reflected. A banana is yellow because when visible
light hits it, the wavelengths that we call yellow are
what bounce back. We've talked about light and color before.
(01:50):
A white light from the sun or a normal light
bulb is composed of wavelengths spanning the entire visible spectrum.
You know your basic ROYGBIV, going from the longest wavelengths
to the shortest. That's red, orange, yellow, green, blue, indigo,
and violet. But when white light shines on a banana peel,
(02:11):
something frankly incredible happens. There's a natural pigment in the
peel called xanthophyl that's chemically programmed to absorb certain wavelengths
of light and reflect others. The dominant wavelength reflected by
xanthophyl is yellow, but the yellow of that banana still
doesn't exist as such. It only starts to exist when
(02:33):
the reflected light from that peel is detected by millions
of color sensing cells in your retinas called cones. Most
humans have three types of cone cells, referred to as red, green,
and blue cones, or more accurately as long, medium, and
short cones, because each are more sensitive to wavelengths of
(02:53):
light in those ranges. The cones send a multitude of
electrical impulses to the brain that data is processed. Each
cone can process one hundred or more different shades of color,
giving your average human a range of a million to
ten million perceivable colors. Some evolutionary biologists think that this
(03:15):
trichromatic color vision evolved in primates to help us spot
colorful berries amongst foliage. But we know that other animals
perceive color differently because they evolved to have different numbers
or types of cones. Dogs, for example, have just two
types of cones, blue and yellow. They don't really perceive
(03:36):
reds and greens. Most birds have four types of cones, red, green, blue,
and ultraviolet, meaning that they can see colors we can't.
Honeybees can see into the infrared part of the spectrum.
Now ultraviolet and infrared light exist, we know they do.
We can observe them in other ways, and we know
(03:58):
that what we call yellow light exists. It's the part
of the electromagnetic spectrum with wavelengths around five hundred and
seventy two five hundred and eighty nanometers or thereabouts. But
other colors that we perceive technically don't exist, like sticky
and blue, or think about magenta. Magenta is what we
(04:21):
see when our brains process a combination of red and
blue light. But red and blue are on opposite ends
of the visible spectrum. There is no wavelength for magenta.
On the flip side of the coin, people with red
green color deficiency might not be able to cite the
difference between red and green apples. For them and for
(04:43):
the best dog in your life, those colors don't exist.
The moral of this color story is this, without our
visual system and without our brains, no color really exists.
It's all in the mind of the ball, which is
enough to make you say dude, And it leads to
(05:06):
a fascinating question. Are there colors within the visible spectrum
that our cones and brains can't see? The so called
impossible colors or forbidden colors break the biological rules of perception.
We're going to get more into impossible colors, but first
(05:27):
we're going to get into a quick break for a
word from our sponsors, and we're back. Thank you sponsors. Okay,
to talk about this, let's start by digging deeper into
the science of color perception. Each of your eyes contains
roughly six million cones concentrated in the center of the retina.
(05:50):
These cones come in those three different varieties types that
pick up on short, medium, or long wavelengths of visible light.
When a cone receives a strong sign in its wavelength zone,
it sends electrical impulses to the brain. The brain's jobs
to combine the millions of electrical signals from each cone
to recreate a composite image of the color that you're
(06:12):
looking at. The brain, of course, is not a computer,
but rather has a complex lump of highly specialized cells
called visual cortex that's responsible for processing the electrical signals
from the cones. We're still learning how these cells work
and work together and with other regions of the brain
(06:33):
to process input from the eyes. But basically the color
related cells are called opponent neurons, and they traditionally have
been separated into two basic types, red green opponent neurons
and blue yellow opponent neurons. They're called opponents because they
function in a binary way. The red green opponent neuron
(06:55):
can signal either green or red, but not both simultaneou,
and the blue yellow opponent neuron can signal either blue
or yellow. I feel obligated to say here that recent
research has indicated that this explanation is actually a culturally
convenient oversimplification of what seems to be a three ish
(07:18):
step brain process that more precisely involves lavender lime opponent
neurons and salmon teele opponent neurons of five varying opponentcies
that can combine to red versus green, and blue versus yellow.
But since I'm not a neuroscientist, oh, let's stick with
the convenient oversimplification for today. Okay, So, opponent neurons how
(07:44):
they basically work is this. When you look at a
pure yellow image, the yellow portion of the blue yellow
opponent neuron is excited and the blue portion is inhibited.
Switch to a pure blue image, and the blue portion
of the opponent neuron is excited while the yellow is inhibited.
And now imagine trying to see an image that's equally
(08:06):
blue and yellow at the same time. The opponent neurons
can't be both excited and inhibited simultaneously. That's why bluish
yellow is an impossible color. The same is true for
reddish green. You might be saying, I know what yellow
and blue look like together. It's green and red and
(08:27):
green make a kind of muddy brown. But that's the
result of mixing two colors together, not of a single
pigment that's equally blue yellow or red green. All the
way back in eighteen oh one, long before scientists knew
about cones and neurons, English physician Thomas Young theorized that
the human eye has three types of color receptors, blue, green,
(08:51):
and red. Young's trichromatic color theory was proven correctsh in
the nineteen sixties, when cone cells, named for their shape,
were to discovered to have special sensitivity to about those wavelengths.
The opponent color theory of perception has been around since
the eighteen seventies, when German physiologist Edvald Herring first postulated
(09:12):
that our vision was ruled by opponent colors across from
each other on the color wheel, with the corset being
red versus green and blue versus yellow. This theory is
simple and intuitive enough that it's still repeated in physiology textbooks,
but in the past five to ten years, neural imaging
has shown that what's going on in our brains is
(09:33):
more complicated. Still, it's functional enough to roll with for today.
So taken together, trichromatic theory and opponent theory argue that
it is impossible for the human eye and mind to
perceive certain colors described as red, green, or blue yellow. However,
(09:55):
as Luigi wisely told us in the nineteen ninety three
Super Mario Brothers Life live action film, nothing's impossible, improbable, unlikely,
but never impossible. In that spirit, in the early nineteen eighties,
visual scientists hewet Crane and Thomas Piantinada designed an experiment
(10:15):
with the goal of tricking the brain into seeing impossible colors.
In their experiment, subjects were instructed to stare at an
image of a vertical red stripe adjacent to a vertical
green stripe. The subject's heads were stabilized with a chin rest,
and their eye movements were tracked by a camera. With
every tiny twitch of a subject's eyes, the redd and
(10:37):
green image was automatically adjusted so that the subject's gaze
remained fixed on the opposing colors. It turned out that
if people stared at adjacent opposing colors for long enough,
the border between them would dissolve and new impossible colors
would emerge. The resulting colors were so new that subjects
(10:59):
had a hard time describing them. By stabilizing the image
to track eye movements, the researchers theorized that different areas
of the eye were being continuously bathed in different wavelengths
of light, causing some opponent neurons to get excited and
others to be inhibited at the same time. But after
several other researchers failed to achieve the same dramatic results,
(11:23):
this experiment was dismissed as a parlor trick. Subjects, instead
of seeing brand new hues of greenish red or bluish yellow,
usually described the blended color as mud brown. Others would
see fields of green with pixelated red dots scattered across it.
Impossible colors became a scientific joke. But then in twenty ten,
(11:47):
biophysicists Vincent Bellick and Brian Zoe published results that they
believed explained why Crane and Piontinita had succeeded where others
had failed. They identified the combination of eye tracking and
luminance or brightness as being key to tricking the brain
into seeing impossible colors. They ran their own experiments in
(12:10):
which subjects were again strapped to a chin rest and
monitored by the latest retinal tracking technology, With the images
stabilized to these subject's eye movements. The researchers played with
the luminance of the two opposing color stripes. When there
was a difference in brightness, the subject's experienced to the
pixelated colors reported in earlier experiments. But when the two
(12:34):
colors were exactly the same brightness, then six out of
seven observers saw impossible colors. Even better, two of them
could see the new colors in their minds for hours
after the experiment was over. While few of us have
a retinal stabilizer in the basement, there are some simpler
exercises that can temporarily trick the brain into seeing the forbidden.
(13:00):
Direct is to stare at an image of two opposing
color squares side by side, each with a white plus
sign in the middle. You can find examples of this online.
What you do is relax and cross your eyes until
the two plus signs merge into one. I tried this
the blue, yellow one was weird, and the red green
(13:22):
one made me laughably uncomfortable. Sort of makes you wonder
what else is out there that we can't see yet.
Today's episode is based on the article how impossible colors
like Stiggy and Blue work on how stuffworks dot Com,
written by Dave Ruse. Brain Stuff is production of iHeartRadio
(13:43):
in partnership with how stuffworks dot Com and is produced
by Tyler Klang. Four more podcasts from my heart Radio.
Visit the iHeartRadio app, Apple Podcasts, or wherever you listen
to your favorite shows
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