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.

One might say that a person causes the property kind to be instantiated when she decides to perform a kind act: She causes there to be a kind act.

But we cannot really say anything like this about static objects. The stone does not cause heaviness to be instantiated, the relationship between stone and heaviness is too close for this. Something heavy comes into existence together with the stone. The stone does not cause its own existence, so it does not cause the instantiation of the property heavy.

Now a tomato is unlike a stone in that it changes (its color turns from green to red) and also unlike a person in that it does not make decisions about how to be. The latter discrepancy seems to me less significant when we are speaking about causality. Considering a tomato plant we can, I believe, say both that it causally produces fruits that eventually mature to the point where they are red (thus causes the property red to be instantiated) and also that it instantiates this property (when parts of it are ripe fruits). I feel less confident about saying this about a tomato that matures on your window sill. It instantiated green yesterday. It instantiates red today. But we would be inclined to say that processes in the tomato, not the tomato itself, (together with external factors such as warmth) caused the change in color. This inclination, however, may be a mere convention: Changes in persons and tomatoes can be caused by processes within them. And there seems to be no deeper reason why we should be prepared to say in the first case, but not in the second, that the change in X was caused by X.

I suspect the point of this question was to see whether you could articulate the idea of a "category error," that is a statement that is syntactically correct but is nonsense because its predicate cannot meaningfully be attributed to its subject.

If this is what the interviewer had in mind, your answer was essentially correct but could have been stronger if you had explained that this was one example of a more general problem. If your interview was at an Oxford college, you probably would have earned bonus points if you had referred to Gilbert Ryle's classic discussion of category errors.

I am unsure exactly why you found this either interesting or baffling. It seems clear that what makes things colored is their tendency to reflect or otherwise emit certain sorts of light. Time doesn't do that, nor could it. As an uncolored phenomenon, time has plenty of company, including space, music, the square root of two, and philosophy.

Now, someone might associate time with a color. People do, after all, report strong "cross-modal" associations between smells and sounds, or tastes and colors. Such a person might describe time as cobalt blue, or as tasting like a turnip. But I think we would be wise not to regard their claims as literally true.

John Locke makes a similar distinction between primary qualities -- roughly your (1) and (2) -- and secondary qualities -- your (3). On his analysis, this is between qualities that actually inhere in things, and qualities that are only in our ideas of things because they are a result of the relation of a thing and our senses.

It seems to me that your question already answers itself. You define meaning 3 as 'sensations of color that this observer experiences'; but your question reads 'which of these three ... am I experiencing?' Perhaps though you meant to use the word 'experience' in two different senses. The first sense (used in definition 3) would be 'the purely mental content that results from the external influence'; the second sense (used in the question) would be 'what is most immediately encountered, rather than inferred.'

Current common sense would tend to answer in the same way you already did, inadvertently. It is the color sensation that I encounter immediately and thus which I experience.

However, consider the following example. A botanist is looking at a tree and she says 'I see that this specimen is lacking potassium.' Must we assume that she sees a particular shade of green and this is for her a sign that indicates the health of the tree? Or can we argue instead that she straightforwardly experiences a tree lacking potassium, and that it would take a special effort to 'back up' from her professional activity, and 'see' just the color green. Philosophers working in the phenomenological tradition are inclined to understand things in the second way.

Even supposing that we accept that analysis, it doesn't yet answer your question. For only a physicist interested in electromagnetic radiation (or perhaps a philosopher pretending to be a physicist!) could be plausibly said to 'experience' a color in that way. As above, a botanist would instead experience a leaf of this or that species, this or that state of health. An interior decorator would see 'just the right' or 'entirely the wrong' paint. Your question assumes (with good reason, and in agreement with Locke) that physics offers the most fundamental and universal account of the 'primary quality' meaning of colour. But that does not necessarily mean it is the most immediately available and useful account.

So, at the very least, we should add a fourth possibility to your list: 'If I am looking at a green leaf, which 'green' am I experiencing?' (4) the green of the leaf.

Instead of answering your question, I just managed to make it more difficult. Apologies.

I don't know how much this will help, but there seem to be three things associated with the color green: the experience of green inside your head; the disposition of certain surfaces to produce that experience, something which is not inside your head though it is defined in terms of something inside your head; and the molecular structure of certain surfaces out there that help to cause experiences of green. Most philosophers agree that all three of these things exist, but they disagree about which of them should be identified with 'the color green'.

This is a classic pr0blem in philosophy, the problem of 'spectrum inversion'. Even if you see blue like I see red, and vice versa, it is very difficult to see how we could ever tell. I cannot see your experiences, and you would use the words 'blue' and 'red' the same way I do, since you were taught to say 'blue' when you saw blue objects and 'red' when you saw red objects, even if your experiences were different. It's interested that spectrum inversion is different from color-blindness. There we can tell, because color blind people can make fewer discriminations.

Suppose that colors have a fourth dimension to which we, humans, are not sensitive. As a matter of fact, I gather that this is true, and that certain birds are now thought to be sensitive to just such an additional dimension. I don't see why this would challenge the existence of the colors we now see. The colors are there, it's just that we're not sensitive to all their aspects.

But this answer assumes a sort of "color objectivism"--that is, that colors are properties out there in the world (certain reflectance patterns perhaps) that obtain independently of our seeing them. Suppose instead that we think of colors more subjectively, as existing in our minds--perhaps as qualitative features of our color-experiences. (I don't think this is the correct way to think of colors, but others do, and certainly some of our color-talk seems to embody this view.) If we think of colors this way (call them "subjective-colors"), then the subjective-colors of the birds--what it's like to have their color-experiences--are quite different from ours. I can't even imagine what it's like to add in sensitivity to that fourth dimension (even after learning a bit about what types of discriminations it allows the birds to make). Perhaps this is like the difference between the experiences of color-blind people and of those who aren't, or perhaps it's more radical. All of this raises interesting questions about how we might (or whether we can) compare experiential qualities across different types of preceptual systems. However, as far as I can see, none of this tells against the existence of our "colors" on this way of speaking--that is, against our subjective-colors. They clearly exist. My computer screen is loaded with objective-colors, and thus my current experience of looking at it is loaded with subjective-colors, even if they are three-dimensional.