It may seem an innocent question, but answering it implies to simultaneously address all aspects that give birth to color. So, before we attempt to do such a thing, let us agree in its basic definition: What is color?
Color is a sensation produced when electromagnetic radiation in the range called visible (having wavelengths roughly between 400 and 700 nm) falls upon human eyes.
The fact that color covers both physic and physiologic phenomena is what makes it exquisitely complex (at least in my opinion). Its objective study led us to colorimetry, the science of measuring what is perhaps the only human sensation we can assign numbers to. And after learning it, one has the sensation (same word again) that we know everything there is to know about it.
Or maybe not.
There's a lot of issues debated around the concept of knowledge. Some people assume that once we know everything physically posible about certain phenomena, then we know everything knowable about it. For instance, if we know every physical detail about the processes triggered when I hit my finger with a hammer, then I should know (according with this line of thought) what sensations (pain in this case) I would feel without the need of having really hammered my finger. Other people, more suspiciously, tend to think that knowing that process in detail and effectively hammering my finger are different things.
This idea seems quite plausible, but in order to accept it we must admit there are certain aspects of sensory experience that cannot be fully described even when we have complete physical information. Therefore, is there any part of that experience that cannot be explained physically, even knowing how our brain works? And if that is true, should we resign to fully knowing it?
In order to shed some light on these questions, several thought experiments have been proposed to reveal the problem in its basics. Mary's room is one of them, specially meaningful because of the question we're asking, proposed for the first time by Frank Jackson in 1982:
Mary is a brilliant scientist who is, for whatever reason, forced to investigate the world from a black and white room via a black and white television monitor. She specializes in the neurophysiology of vision and acquires all the physical information there is to obtain about what goes on when we see ripe tomatoes, or the sky, and use terms like "red", "blue", and so on. She discovers, for example, just which wavelength combinations from the sky stimulate the retina, and exactly how this produces via the central nervous system the contraction of the vocal cords and expulsion of air from the lungs that results in the uttering of the sentence "The sky is blue". What will happen when Mary is released from her black and white room? Will she learn anything or not?
It's astonishing the outcome of this experiment is far from being resolved. There is two points of view so far: those who supports physicalism, that is, the idea that physics is enough to explain everything, and the ones who subscribe to epiphenomenalism, according to which mental states may be caused by physical and biochemical events but their existence is something different from physics, including subjective conscious experiences (qualia). For the former, Mary learns nothing new after being released from her confinement; for the latter, she may exclaim "Wow! So that is color?"
I consider myself unable to follow any of these theories of philosophy of the mind, but this subtle distinction between what we can know from our interactions with the physical world and what we actually feel, may serve even a little to try to answer our original question. In fact we will put physics (considered well known) momentarily aside and dive into physiology, where there seems to be a lot to discover.
The colorimetric point of view: "normal" colors
The usual tool we use to analyze colors is the map known as chromaticity diagram, which conveniently represents in a plane all colors humans with normal vision can see. That said, we may be begging the question if we say this diagram contains every existing color, but let's move forward for a while.
As we recall, in this diagram every color shown is unique except for its brightness; for instance, browns are covered by oranges, grays by white, etc. Every color located at the curved border are pure monochromatic colors, the ones we see in a rainbow. The straight line at the bottom is the purple line, and represents those obtained mixing the opposite colors of the visual spectrum. The lightly irregular shape stroked in white is the Pointer's gamut, an approximation of the gamut of real surface colors (either natural or man-made objects) as can be seen by the human eye.
According to this description, every point inside de chromaticity diagram but outside Pointer's gamut is a color that:
- Was already experienced by some human being in the form of light, or:
- Eventually exists (in nature or synthesized) but yet (strictly speaking) hasn't been experienced by any human being whatsoever.
The latter may be called "colors yet unseen", but I suspect it's illicit to call them "undiscovered colors" because we know where they are and how to eventually synthesize them. As en axample, in 2015 it was discovered by accident at the Oregon University a new compund based on manganese oxide that when heated at 1200 ºC (around 2000 ºF) produces a crystalline structure strongly absorbing red and green wavelengths, giving as result a deep blue never seen before in a compound. Nevertheless, its color can perfectly be located in our diagram.
The physiologic point of view: "chimerical" colors
It is well known that current colorimetry is based on "typical response of a normal human being at normal conditions", derived from the fundamental experiments carried out by David Wright and John Guild at the end of the 1920's, and standardized by CIE in 1931. But this definition suggests a way to reach "unofficial" colors, unaccessible to standard colorimetry: keeping the normal human, what happens if his vision conditions are not quite "normal"? Is it possible, by changing typical observation conditions, to perceive "irreal" colors?
The standard way of achieving that altered condition is to take advantage of a feature of our cone cells, located at the retina and responsible of our color vision. You may know that, in the presence of some excitation for a long time, cones become "saturated" and are temporarily insensitive to the region of the spectrum they usually operate, which produce an afterimage several seconds after suppressing that excitation. Here I have a demo for that.
Instructions: Click on start and watch the blue circle at the middle of the flag. After 30 seconds, flag will turn into a white rectangle. During a short period an afterimage of the Argentine flag will be seen. Click on restart to repeat the experiment.
This effect, itself an evidence of the color opponent process, can be explained in this way: red-sensitive cones lose sensitivity after many seconds of continuous exposure; when white (demanding participation of all three types of cone cells) is suddenly shown, the temporary lack of response of those cone cells subtracts some red, leaving light blue (technically cyan) instead. A similar process occurs with central blue, appearing as yellow.
This saturation condition is carefully avoided in the definition of the standard observer XYZ curves on which colorimetry is based on, hence sensation produced in this way may in some circumstances be outside our chromaticity diagram. In fact, certain colors can only be observed making use and even abuse of this effect: they are chimerical colors. Take a look at the following examples.
These colors receive their names from the river Styx at the Hades, the underworld in greek mythology. They are as dark as black but extremely saturated. Typical example is stygian blue:
Instructions: Click on start and fix your eyes on the cross over the yellow circle. After 30 seconds, the gray square will turn to black. During a short period you will see at the center a blue circle very dark but yet very saturated. Click on restart to repeat the experiment.
These colors appear to glow even if viewed on paper. Here we show self-luminous red, which is kind of a red that seems to be brighter than white:
Instructions: Click on start and fix your eyes on the cross over the green circle. After 30 seconds, the gray square will turn to white. During a short period you will see at the center a reddish circle brighter than surrounding white. Click on restart to repeat the experiment.
They are colors impossible high saturated. Here we show hyperbolic orange, an orange more saturated than the monitor's.
Instructions: Click on start and fix your eyes on the cross over the cyan circle. After 30 seconds, the gray square will turn to orange. During a short period you will see at the center an orange circle more saturated than surrounding orange. Click on restart to repeat the experiment.
Taking things to the extreme: "impossible" colors
While chimerical colors are produced by means of afterimages and takes advantage of the color opponent process, impossible colors try to "defeat" it. Let's recall that information produced by cone cells is processed in antagonistic manner in three different channels: red vs. green, yellow vs. blue y white vs. black. It is generally accepted that this process is hard-wired, determined by our biology and thus "built-in". As a consequence, every color gets its location inside these channels between "adversary" colors; this explains why we cannot normally expect to see "greenish red", "yellowish blue", etc. However, despite the fact that not all researchers agree, there are people who claim to see those colors forbidden to normal vision.
A method often proposed (but it hardly works, I'll explain later why) is to just force our eyes to see red and green at the same time. Simply try to cross your eyes and force the squares below until their centers match:
It doesn't work for me, and maybe for you neither. At most I barely see some translucent red over green, or a fast alternating pattern of red and green, but no "greenish red" whatsoever.
The earlier reference to those anomalous colors I found is a research done in 1983 by Hewitt D. Crane and Thomas P. Piantanida from SRI International, Menlo Park, California. They had several subjects look at side-by-side stripes of red and green or yellow and blue. Their apparatus tracked their subjects’ eye positions and moved mirrors to keep those stripes stabilized (frozen in place on each subject’s retina despite all the continual little movements of the eye). Image stabilization can lead to many interesting effects, such as an image seeming to break into pieces that wax and wane in visibility. Their experimental subjects saw the border between the two opponent colors vanished and mixed across the border in common. Some subjects reported seeing the forbidden reddish greens and yellowish blues. This may explain why just "crossing our eyes" in the above experiment isn't enough: it's practically impossible to get an stable superposition.
This result has been controversial, mainly because other researchers were not able to reproduce the experiment. But in 2001 Vincent Billock and Brian Tsou, from Wright-Patterson Air Force Base in Ohio, presented a paper where they claim that Crane and Piantanida experimental results can be obtained if opponent colors used were "equiluminant". To get an idea, two colors are equiluminant if switching them very rapidly produces the least impression of flickering.
If new studies confirm this, the hypotesis of a hard-wired color opponent process should be revised in favor of a soft-wired cortical model. In other words, opposite colors could be a product of our "software" instead of our "hardware", and as we know, software can be hacked...
-  Accidental discovery produces durable new blue pigment, Oregon State University.
-  On Seeing Reddish Green and Yellowish Blue, Science, edición 4615, págs. 1078-1080.
-  Perception of Forbidden Colors in Retinally Stabilized Equiluminant Images: An Indication of Soft-wired Cortical Color Opponency? Vincent Billock, Gerald Gleason y Brian Tsou, Journal of the Optical Society of America, Vol 18, págs 2398-2403.