Does color exist?
Well, like many a philosophical question, I think it comes down to what you mean by exist and what you mean by color. I think people generally have 3 things in mind when they speak of color:
- The human perception of color
- The property of electromagnetic radiation
- The subjective sensation of color
So let me define clearly what I mean by each of those, and then talk about in what sense they exist.
In the first sense, I refer to the fact that there is a property of things that humans perceive that we call color. I assert that I can prove that this exists pretty easily, because, hey, traffic lights work. What I mean is that I can prove the existence of color as a property that (most) humans can perceive as much as I can prove the existence of any other property (like size, shape, texture), because I can tell by observing people react to this property and use it in cases like the traffic light to coordinate their behavior. So, the hypothesis that a property called color exists which relates to the perceptions of different people is pretty well supported. The point here is that even someone who is color blind, or even completely blind can still surmise in this way that colors exist. So far, we've said that some property called color exists and that there is a correspondence between how many people sense it, though this does not say their sensations are the same.
The second sense in which people use color is with respect to light. We refer to light as having a color, corresponding to the mixture of wavelengths present in that electromagnetic radiation. There are both spectral colors, which correspond to light with only one narrow range of wavelengths (e.g. red, green, blue), and then there are other colors that are mixtures of several wavelengths in certain proportions (e.g. cyan and magenta). This second property, the spectrum of the light, can again be verified with a spectrometer without the faculty of human color perception. This, of course, is something humans discovered much later than the first sense, but the reason we call this color is that we can observe a correlation between the properties of electromagnetic waves leaving an object and the color that people say they perceive. Simply put, if we replace a the red light in a traffic signal with another source of light with the same spectral characteristics (something with a strong peak in the 600-700 nm range or so), people will react in approximately the same way. In so far as this is true, we can ascribe the spectral properties of light coming from an object as the cause of the perception of color by humans, without really having to know anything about HOW humans process the light, so long as it seems plausible that they could.
So, it seems that in the first two senses, color exists, in so far as it can be empirically verified through observation. Objects emit light, and that light induces a particular perception in most humans. Generally, though, when people ask about color in a philosophical context, though, they mean the third sense.
By the subjective sensation of color, I mean the sense of what red or blue "looks like" when a person with color sensitive vision looks at an object. This, I suppose, is what is meant by qualia. Certainly it's clear that people believe they have such a sensation, again because this is the best explanation of the fact that they claim having such a sensation and it correlates in a single individual with the other senses in which we use the term color (ie. someone will report that something looks red when we show them an object with the right light spectrum that other people also say is red). Generally the question is then whether the subjective sense of red is the same for all individuals, whether red looks the same to me as it does to you.
When we've narrowed the question down this far it clearly becomes hard to answer. If we are willing to make the materialist hypothesis that there is a one to one relationship between the physical state of the body and brain and the subjective perceptions of a person's mind, then this question is answerable through scientific testing. I'm fairly certain that at the moment we don't really know enough about the brain to give a definitive answer, but in the future it's likely we will be able to. If, however, you are not willing to make the materialist hypothesis, then without some further assumptions there is no way of knowing whether colors are perceived the same way in every person in the empirical, scientific sense. With other metaphysical assumptions, you might be able to make some conclusion, but otherwise you're stuck.
Personally, I don't subscribe to any other metaphysical definition of existence. I would say I know that I get a certain sensation when I see a color. I cannot doubt that because it is simply there in my mind, undeniably, as much as any other of my sensations or ideas. As to how color exists in the minds of others, I know that there is this correspondence between my sensation of colors and others' apparent perception of colors (as they report it, and as reflected through their actions). It is that correspondence that exists, and that's what I really mean when I say that color exists. To me, any further, untestable sort of existence is nonsense.
So, if you're reading this and you just think it's pointless, useless, or redundant drivel, then let me know. I may consider removing it.