Could there be colours we haven't yet seen?

A lot obviously depends on what you mean by "colour." (I'll assume "we" means human beings taken collectively over time and space!) If you take a physicalist/reductionist approach and identify colours with some property or properties of light or electromagnetic radiation within the visible spectrum, then, roughly, we might say "no" -- very plausibly light of every frequency (say) in the visible spectrum has causally interacted with the visual system of human beings causing a relevant visual experience etc. But if visual experience is more complex than that -- and it is -- perhaps there are all sorts of combinations of frequencies (and intensities etc) that we have not yet come across, which might even produce different kinds of visual experiences than we've had to date, turning that no into a yes. And surely that initial no is a kind of boring answer, and probably you have in mind something more expansive in asking your question. Perhaps (for example) we can imagine colours corresponding to parts of the EM spectrum that are not (currently) visible to us, b/c our eyes aren't wired correctly; perhaps we can imagine subjective accounts to colour, treating colours not as properties of light but of conscious experience itself. On both those scenarios the more plausible answer to your question seems to be yes, at least within the limits of our imagination -- in the former we know there are all sorts of EM radiation outside of currently visible radiation, and anyway who knows how our visual systems might evolve naturally over time, or artificially via technology; and re the latter, who knows or could tell what kinds of conscious experience are possible? Since I personally reject objectivist/realist accounts of colours, I am far more inclined to the yes answer to your question -- but ultimately the question itself really requires grappling with the competing analyses of colour before getting an answer ...

Hope that's useful!

A lot depends on who "we" are. Suppose we are complete achromats, so that we only see "achromatic colours", greys, blacks and whites. Now it is clear enough that there is a colour "we" haven't seen. Red is an example, and so are all the other chromatic colours. Now what if "we" is the whole human race at all times. We can imagine that we take the set of all the colours that anyone has ever seen. Question: are there more? Wittgenstein is someone who takes a negative line on this. Are they colours that others might see which we do not? '. . . [W]e would still not be forced to recognize that they see colours that we do not see. There is, after all, no commonly accepted criterion for what is a colour, unless it is one of our colours' (Wittgenstein, Remarks on Colour I, 14). We do not use colour language just by pointing to colour samples and naming them. It takes more to be talking about colours than that (III, 58). The idea of little coloured patches of colour is not a "more fundamental" idea of colour than our actual idea. We have to ask questions such as 'How do we compare colours?' to be sure we do have the concept of colour at all. When we say bees see in the ultraviolet, which they do, are we forced to say that what they are seeing there is colours? How would we compare the colour violet with what the bees "see" - if seeing is what it is?

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