Is there an objective test which can be done, perhaps going over the entire visible light spectrum, that can determine all possible colors that humans can see?
Is it even possible to speculate on spectrums invisible to humans? Maybe X-rays or Gamma rays have a color but the eye cannot see it. 
:eek:
I asked a similar question nearly 7 years ago (wow!) and it might be a good starting point until some others comment.
What we think of as “color” is an artifact of how the human eye functions.
Basically the human eye has three different sensors that respond to three different wavelengths of light. Each different combination of “tristimuli” is perceived as a different color.
Wavelengths outside the visible spectrum don’t stimulate any of the three sensors and so don’t have any color. And some colors we can see don’t correspond to any pure wavelength of light.
Humans have three kinds of color receptors in their retinas. (Most mammals have two.) Most other vertebrates, including birds, reptiles, amphibians, and fish, have four kinds of color receptors, which include some sensitivity in the ultraviolet. So yes, most other vertebrates can see colors that we cannot, and which are as incomprehensible to us as “chartreuse” would be to a color-blind person.
It has been found that some birds that seem to have quite dull plumage to us, show up brightly in ultraviolet. They look brilliantly colored to other birds, but in a color we cannot visualize at all.
Are you basically asking the same question as, given the visible spectrum–that is, all possible wavelengths between what are commonly identified as the upper and lower limit–are there any gaps, any tiny subranges in the middle which humans (or most humans) cannot see?
I am certain that these types of experiments have been done. Since we have not heard about them, I would bet that the answer is that a human with normal color vision can in fact see all wavelengths in the range. That there are no gaps.
But I, as a red-green “colorblind” person, cannot disinguish the color of all wavelengths even though I can see the light. If you illuminated a room with narrow-wavenlength of light of the right frequency, I would not be able to tell you the color, but it would not just look dark to me.
No, this does not make sense. Color is something which the human vision system imposes on the spectrum. The spectrum is just wavelenghts of continuously varying lengths. There is nothing really in the light itself to distinguish blue from green. The colors are a result of the way the combination of the retina and visual cortex work together to interpet what is seen.
For instance if the two wavelengths 450 nm (nanometers) and 480 nm are only 30 nm different yet both are seen as blue (I am guestimating the actual values.) While a 510 nm wavelength beam would be seen as green, even it is only another 30 nm different from the 480 nm (blue) wavelength.
If intelligent birds ruled the world, their chicks would learn in elementary school about the four primary colors and how they can be mixed together to form all the others. And their computer monitors would be designed with 4 different types of phospor instead of three … .
>Humans have three kinds of color receptors in their retinas.
We actually have receptors that are sensitive to four different colors, but one of these is not wired to use in distinguishing color.
A range of colors is, in technical work, called a “gamut”. People that work on color science have certainly seen the whole range of extreme colors and many colors between them. Because we use a ratio between three numbers to sense color, there are two degrees of freedom in color, and I don’t think you can ever do anything with all possible combinations of two degrees of freedom. That is, if you run a pin along a length of thread, you have touched the pin to all points along the thread, but you can’t trace a path on a tabletop to touch all its points because you’d have to go back and forth along paths with infinitesimal separations.
I once read that if the color spectrum is laid down across the US our visible spectrum that humans can see would only be like a foot long.
Yes, this is what I’m talking about. Thank you for summarizing it so succinctly.
Also, to expand further, could there be, as others have mentioned, more spectrums that we are simply not privy to because of the limitations of our eyes? I don’t mean things that glow in ultraviolet light, because ultimately what we see is simply a different shade of the colors we already have come to know. But some color that we cannot fathom? Is there evidence of that?
To extend your example, maybe all humans are colorblind to certain colors. But the difference is, people who have red-green colorblind know there exists colors they cannot see and know those colors are radically different from the colors they are able to detect. The possibilities just blow my mind
That’s not a good example. What colour is “chartreuse”?
Again, “color” is about how a sensor (in our case, the eye) responds to light, not so much about the light itself. So your question boils down to “Could there exist sensors which operate on different principles than our eyes?”. And the answer to that is, of course, yes. Most animals have significantly different color vision than we do, and we can manufacture sensors which have many possibilities beyond that.
You’re going to have to define what you mean by “the whole spectrum”, as well as how you’re measuring “length” along that spectrum. There is no known bound on how long wavelengths can get, nor on how high frequencies can get, so you’re effectively talking about compressing an infinite-length line down to a finite length.
What about colors like Pink or Brown? I don’t think either of one of them is in the color spectrum
This one, although it may apparently refer to two different shades of yellow-green. And I had always thought it was reddish for some reason…
Valete,
Vox Imperatoris
It is the color precisely halfway between green and yellow, so it is 50% green and 50% yellow. It is common enough to be a named color in HTML, and hex color number #7FFF00.
They are tints of colors; pink is a lightened red (mixed with white), while brown is a darkened orange.
Colours are just convenient labels used by our brains to identify sensations - there are wavelengths that could be visible to us as ‘light’, but are not visible, and I suppose it’s possible that our brains could in that scenario have developed differently from how they did, and would have a wider range of labels for the wider range of sensations.
But it didn’t happen that way. There aren’t any more colours than those we see, because the concept of colour is defined by what we can see, not the other way around.
Pink and brown, along with such colors as black, white, and reddish purple, are not spectral colors. They involve mixtures of colored light and white light, which itself is a mixture of more or less all the colors of visible light.
Pink is a tint of red–that is, red light mixed with white light. All three cone types are being stimulated by the mix of colors in white light, but red the most.
If you play around with a hex color tool, you can see that brown is essentially a dark orange or yellow. (Why “dark yellow” is so hard to visualize is probably a question for another thread.) We can get that from stimulating red and green cones more or less equally with either yellow light or a mix of red and green light, but at low levels.
Colors like grey, black, and white are simply white light at varying intensities. You can see lots of mixed colors that aren’t simple spectral colors.
Purple is a non-spectral color that I hesitate to talk much about. It’s a mixture of blue/violet light and red light, but I can never fathom exactly why our visual system would “close the loop” between the highest- and lowest- frequency colors by making their mixture something that’s qualitatively similar to both of them. If anyone can make me see the light re: this problem, I’d finally have peace of mind.
(Heh. My feelings on purple are nicely approximated thus: 
)
Kind of. According to the wiki, the web color chartreuse is exactly half-yellow and half-green, but the traditional color is more yellow. So if something is actually called chartreuse, it is likely to be more on the yellow side than the green side.
Valete,
Vox Imperatoris
Humans see all colors pretty much by definition.
It’s all electromagnetic radiation from down in the radio waves to up in the gamma rays. We can’t directly sense most of the spectrum but we can sense the section with wavelengths between 380 and 760 nanometers. That’s visible light.
Color is a function of being visible. There’s a lot of electromagnetic radiation we can’t see but if we can’t see it then it doesn’t have any color. Microwaves or x-rays are electromagnetic radiation but they aren’t colors.
I’ve wondered some about this, myself. Is it possibly related to the fact that the highest-frequency light we can see is close to an octave of the lowest-frequency light we can see?
I have long suspected that this is due to the highest violets being almost half the wavelength (or twice the frequency) of the lowest reds.
With sound, we can perceive multiple doublings of frequency, or octaves. When tuning a musical instrument, A is 440 Hz, but its double in frequency at 880 Hz is also an A, and so on. There’s some common quality perceived in a sound and its octaves.
We traditionally speak of sound in frequency, but we traditionally speak of light in wavelength. Violet gives way to ultraviolet as wavelengths shorten past 400 nanometres (nm). Red gives way to infrared as wavelengths lengthen past 700 nm. This, we can’t quite see an octave in light. But we get close, and I wonder whether purples take advantage of two violet wavelengths being almost the same as one red wavelength.
I really should get some LEDs of different colours and start experimenting…