"Opposite" is not in this connection a very well-defined word. "Complementary" is more precise, but then we should inquire: physical additive complementary, i.e. such as to cancel the test colour in light superposition and produce neutral or white; physical subtractive complementary, i.e. such as to cancel the test colour in pigment mixing and produce neutral or black; psychologically complementary - it is unclear what this would mean, but it could have to do with the placing of the test in a colour space based on the psychologycal "unitary" hues, i.e. those that do not look as though they contain a "trace" of any other hue in the space.

There are some interesting studies of brown, and one of them (I think) is my own, in Jonathan Westphal, Colour: A Philosophical Introduction, Blackwell, Oxford, 1991 - the chapter on "Brown". Are we allowed to sound our own trumpets on this website? I'm not sure, but anyway this might get you started. The thing to remember is that brown surfaces have roughly the same reflectance as yellow ones, but they are quite a bit dimmer. It is as though brown is really a low-reflectance yellow, so one thing you might try is to see what the afterimage colour of brown is - is it similar to the violet afterimage of yellow, but dimmer? Afterimage complementaries give you yet another - psychological - sense of "opposite"!

The fact is that brown is not neutral. At its reflectance level, grey is the neutral. But I'm not sure what you had in mind with the phrase "representing the balance of primary/secondary/tertiary (etc.) colors. I think you may be mixing the metaphysical primary/secondary quality distinction with the physical distinction between "additive" and "subtractive" primaries.

This is a fairly frequent concern. The correct answer is that there is a sense of "colours" in which black and white are not colours (they are not chromatic colours) and a sense in which they are colours (they are achromatic colours). So if we count the achromatic colours (black, white and grey) as colours, then black and white are colours. (Brown is an interesting case, as it is a colour which is partially achromatic.) In the same way, we can ask whether zero and infinity are numbers. Usually they are treated as numbers, and they have their own mathematical symbols. We can manipulate them in calculations and so forth. But in another sense "zero" denotes the absence of a number, and so does the symbol for an infinite number. Q: "How many chickens were there in the kitchen?" A: "A number." Q: "What is the number?" A: "Zero"! Aristotle's view was that the smallest number is two, as one of something is not a number of somethings. "There were a number of people there." How many?" "One." In this sense two is the first crowd-like or milling number. One won't mill around. Logicians face the same difficulty in explaining that in their sense "some" means only "at least one".

The situation is that colours arrange themselves into three dimensions: saturation, hue, and brightness. Hue is colourfulness, the colourfulness of red, yellow, blue, green and so on, and colourfulness does not include black, white and grey. Colourfulness is the circling hue dimension at maximum saturation, and the achromatic colours lie in their own vertical dimension at the center of the solid whose surface is this colourfulness or saturation. White has zero saturation, and we make other pigments of various chromatic colours less saturated - paler - by mixing in white pigment. (It is an interesting question why this concept - paleness - has a "special relationship" only with white.)

So at the end of the day the fact is that in one way black and white behave as colours, and in another way they work to create diminutions and absences of colour. Wittgenstein was right (in his Remarks on Colour) to see a puzzling element of necessity, a necessity as hard as logical necessity, in these striking facts.

First, take a look at Question 2384 and its answers, which are closely related to your question. Your question is related to what is called the "inverted spectrum", a philosophical puzzle posed by John Locke, one version of which is this: Is it possible that objects that have the color you describe as "red" are seen by me as if they had the color you describe as "green", even though I also describe them as red, and vice versa? Posing the problem is difficult; e.g., objects arguably don't "have" colors, but reflect light of certain wavelengths, which are perceived by us as certain colors. "Is the color that I perceive as, and call, red the same as the color that you perceive as what I call blue?" is another way of posing the puzzle. Part of the problem is that there doesn't seem to be any way to decide what the answer is (if, indeed, it has an answer). What experiment would decide between these? Perhaps such color-perceptions (more generally, what are called "qualia") are such that a functionally complete theory of the mind (or brain) would not enable us to distinguish between them. For more on this, see the Stanford Encyclopedia of Philosophy article on "Inverted Qualia".

There are two different, but related, issues here, on neither of which is there universal agreement among philosophers (but, then again, is there ever?).

First, there's "Molyneux's problem": Can a person born blind who later gains sight distinguish a cube from a sphere merely by sight (assuming the person could distinguish between them by touch)? There's some empirical evidence that the answer is "no". The psychologist Richard Gregory has investigated this.

But closer to your specific question is the philosopher Frank Jackson's thought experiment about "Mary", a color scientist who lives in a completely black-and-white world but who is the world's foremost expert on color perception. She has never experienced red. Would she learn anything if she experienced it for the first time? I.e., is there anything "phenomenal" to the experience of red over and above what physics can tell us? Jackson originally argued that there was, i.e., that Mary would learn something from the experience of red, namely, what it's like to see red, but he has recently changed his mind. The novelist David Lodge has explored the Mary story in his novel Thinks....

For more on Jackson's thought experiment, see the anthology edited by Peter Ludlow, Y. Nagasawa, and D. Stoljar, There's Something about Mary (MIT Press, 2004).

Do you think that colours emitted by neon gas have a particular neon quality? I'm not sure. But your question could very well be asked of the metallic colours, such as silver and gold. They are not "made" by any combination of primaries, so how are they made?

There are objective scientific tests which show that we don't all see colours the same, such as the Ishihara test for colour vision. Most people don't even see the same "colours" out of both eyes. For many people the left eye might see things more saturated than the right.

The question should also perhaps be refined a bit. Shouldn't it be formulated as whether we see things (objects, surfaces, volumes etc.) in the same colours? "Do we see colours the same?" as it stands seems to mean, "Do you see red as I see red?" But this presupposes that we are both seeing red, and then the question seems to ask whether we see it the same way, for example with the same degree of saturation or exactly as blue.

This is a much discussed question, which often appears in the guise of the "inverted spectrum hypothesis": One might wonder whether some other person sees what you see as red the way you see green, etc. It turns out it can't be quite that simple, but one might nonetheless wonder whether we do all see colors the same way.

In fact, Ned Block has argued that there is some empirical evidence that we don't all see colors the same way. (See this paper.) It goes without saying that this is very controversial.

This is one of the issues that perplexed Wittgenstein in his work on colour. The fact that there just seem to be the colours that there are looks like a synthetic a priori proposition, necessary in some sense, but describing matters of fact. Yet colours have often been taken to be good examples of what empiricist philosophers called secondary qualities, features of reality that may be experienced in different ways by different people. How then can we lay down as a rule that there will be no other colours? Wittgenstein argues to a degree that we have this colour system and it is fixed in the way it is right now, and so thinking about new colours does not really make sense within the framework of that colour system. But then he also argues that we could not make sense of the idea of people going to the moon, given our system of physics, and not only can we make sense of this, it has even happened. It rather depends on whether you think that our colour system is an interconnected network of meanings that cannot be broken or changed without radical alteration to what we mean by colour itself, or whether you think it is more like a scientific system. What is fascinating about colour is that it seems to be both, something that Wittgenstein invites us to reflect on.

Neon red is simply red light emitted from an enclosed clear transparent vacuum tube containing the element neon, to which an electrical current is applied. Neon actually emits a red-orange color. Other colors can be obtained by introducing other gases in the enclosure and also by changing the color of the tube. What you may be asking about is the distinctive glow emitted from a neon tube. The red of a neon tube appears different from the red of a painted surface, for example. Different materials emit or reflect red light in different ways, and our visual system is sensitive to many of those differences.

I think you've pretty much answered your own question. You see (get it?) that light could exist even in the absence of any creatures sensitive to it. And of course in such a situation, there would be no one and nothing experiencing the light. So is anything visible? "Visible," like many English words that end in "ible," "able," "uble," or "ile," picks out what philosophers call a "disposition" -- a condition of being ready, so to speak, to cause certain things to happen, or to undergo certain changes, if certain conditions are met. Salt is soluble -- that means that if it's put into a pot of water, then it will dissolve. Waterford crystal is fragile -- if you drop it, it will break. Similarly, to call an object visible is to say that if it is illuminated, and if a creature that is sensitive to light points its sensitive parts toward the object, then the object will cause the creature to have visual experiences (by bouncing the light onto the creature's eyes in a particular pattern).

The thing about dispositional properties is that objects can have them even if and while the "activating" conditions are not being met. The salt is soluble even while it's sitting in the carton (that's why you want to store it in a dry place -- if it weren't soluble, it wouldn't matter.) And the Waterford crystal is fragile even while it's sitting on the dining table (that's why you need to be careful with it. In fact, best not to use the good crystal at all.) Similarly -- and here comes the answer to your question, finally: things in the universe can be visible even if there's no one around for them to cause visual experiences in.

Now you wanted a little science, so here's a little science. It's an interesting question, part scientific, and part philosophical, whether color is a dispositional property of things. It turns out that color perception is amazingly complicated, and that it is not just a reaction to or detection of simple physical property. The physical property of objects that comes closest to being the objective basis of color is "spectral reflectance" -- a dispositional property of surfaces to reflect incident light in different patterns of wavelengths. But there are different combinations of wavelengths that will be perceived by human beings as the same hue. So what's color? Is it just the dispostional property of producing the effect of color perception in human beings (or other perceivers)? But in that case, the set of physical structures that have the property of being, say, teal is not going to be picked out by some objective physical property they all share, but rather by reference to the "teal-effects" they produce in us. That's different than the situation is with light, and it's lead some philosophers to conclude that color is really in the eye of the beholder. I think that view is wrong, because I think a set of physical structures still has something objectively in common if they all have a common power. But there's a lot of disagreement about this whole thing.

If you'd like to learn the details of the color debate, as well as some serious color science, here are two recommendations for further reading: the entry on "Color" in the Stanford Encyclopedia of Philosophy: http://plato.stanford.edu/entries/color/ and a book called Color for Philosophers by C. L. Hardin.