The problem of qualia is in itself a very old one, but it is expressed in new terms.  My impression is that the actual word ‘qualia’ only began to be widely used (as a hot new concept) in the 1970s.  The question of whether the colours you experience in your mind are the same as the ones I experience in mine, on the other hand, goes back a long way. I’m not aware of any ancient discussions, though I should not be at all surprised to hear that there is one in, say, Sextus Empiricus (if you know one please mention it): I think the first serious philosophical exposition of the issue is Locke’s in the Essay Concerning Human Understanding:

“Neither would it carry any imputation of falsehood to our simple ideas, if by the different structure of our organs, it were so ordered, that the same object should produce in several men’s minds different ideas at the same time; e.g. If the idea, that a violet produced in one man’s mind by his eyes, were the same that a marigold produces in another man’s, and vice versa. For since this could never be known: because one man’s mind could not pass into another man’s body, to perceive, what appearances were produced by those organs; neither the ideas hereby, nor the names, would be at all confounded, or any falsehood be in either. For all things, that had the texture of a violet, producing constantly the idea, which he called blue, and those that had the texture of a marigold, producing constantly the idea, which he as constantly called yellow, whatever those appearances were in his mind; he would be able as regularly to distinguish things for his use by those appearances, and understand, and signify those distinctions, marked by the names blue and yellow, as if the appearances, or ideas in his mind, received from those two flowers, were exactly the same, with the ideas in other men’s minds.”

Interestingly, Locke chose colours which are (near enough) opposites on the spectrum; this inverted spectrum form of the case has been highly popular in recent decades.  It’s remarkable that Locke put the problem in this sophisticated form; he managed to leap to a twentieth-century outlook from a standing start, in a way. It’s also surprising that he got in so early: he was, after all, writing less than twenty years after the idea of the spectrum was first put forward by Isaac Newton. It’s not surprising that Locke should know about the spectrum; he was an enthusiastic supporter of Newton’s ideas, and somewhat distressed by his own inability to follow them in the original. Newton, no courter of popularity, deliberately expressed his theories in terms that were hard for the layman, and scientifically speaking, that’s what Locke was. Alas, it seems the gap between science and philosophy was already apparent even before science had properly achieved a separate existence: Newton would still have called himself a natural philosopher, I think, not a scientist.

It’s hard to be completely sure that Locke did deliberately pick colours that were opposite on the spectrum – he doesn’t say so, or call attention to their opposition (there might even be some room for debate about whether  ‘blue’ and ‘yellow are really opposite) but it does seem at least that he felt that strongly contrasting colours provided  a good example, and in that respect at least he anticipated many future discussions. The reason so many modern  theorists like the idea is that they believe a switch of non-opposite colour qualia would be detectable, because the spectrum would no longer be coherent, while inverting the whole thing preserves all the relationships intact and so leaves the change undetectable. Myself, I think this argument is a mistake, inadvertently transferring to qualia the spectral structure which actually belongs to the objective counterparts of colour qualia. The qualia themselves have to be completely indistinguishable, so it doesn’t matter whether we replace yellow qualia with violet or orange ones, or for that matter, with the quale of the smell of violets.

Strangely enough though Locke was not really interested in the problem; on the contrary, he set it out only because he was seeking to dismiss it as an irrelevance. His aim, in context, was to argue that simple perceptions cannot be wrong, and the possibility of inconsistent colour judgements – one person seeing blue where another saw yellow – seemed to provide a potential counter-argument which he needed to eliminate. If one person sees red where another sees green, surely at least one of them must be wrong? Locke’s strategy was to admit that different people might have different ideas for the same percept (nowadays we would probably refer to these subjective ideas of percepts as qualia), but to argue that it doesn’t matter because they will always agree about which colour is, in fact yellow, so it can’t properly be said that their ideas are wrong. Locke, we can say, was implicitly arguing that qualia are not worth worrying about, even for philosophical purposes.

This ‘so what?’ line of thought is still perfectly tenable. We could argue that two people looking at the same rose will not only agree that it is red, but also concur that they are both experiencing red qualia; so the fact that inwardly their experiences might differ is literally of no significance – obviously of no practical significance, but arguably also metaphysically nugatory. I don’t know of anyone who espouses this disengaged kind of scepticism, though; more normally people who think qualia don’t matter go on to argue that they don’t exist, either. Perhaps the importance we attach to the issue is a sign of how our attitudes to consciousness have changed: it was itself a matter of no great importance or interest to Locke.  I believe consciousness acquired new importance with the advent of serious computers, when it became necessary to find some quality  with which we could differentiate ourselves from machines. Subjective experience fit the bill nicely.