Is it conceivable that there exists an as yet undiscovered colour? The obvious answer is “obviously not” – all one has to do is traverse the electromagnetic spectrum from infra-red to ultraviolet, and one has seen them all.
But if one takes a subjective rather than a purely scientific approach to this question, the answer becomes perhaps a little less clear. Who has not seen some stunning colour in a painting or a sunset that seems entirely its own – and is not at all to be found in the Microsoft Paint palette? One may argue that this is merely a psychological trick which the composition of the painting or the brilliance of the sunset plays on us; but then, we are interested particularly on the subjective effect of the colours.
This encourages us to believe that maybe there are other colours which can be perceived, if one seeks to enlist other agents in support of the mere electromagnetic wave. The first things that springs to mind are psychoactive drugs, with which your correspondent admits a lack of familiarity. Also, they are “cheating” in the sense that any novel chromatological experiences are merely simulated internally rather than stimulated externally, although the boundaries are admittedly blurry.
So who knows – maybe there is some unique combination of electromagnetic waves of various frequencies, made to perform some elaborate microscopic dance upon one’s retinas, which, when the experimental conditions are right (test subject 1 m under water, eyeballs gently vibrated), necessarily leads to the perception of an entirely new colour, which no man had theretofore ever experienced.