To me this point is patently obvious but lots of people I know disagree. Also this means the primary/secondary quality distinction is legitimate.

Consider yellow, for example. We may try to define it, by describing its physical equivalent; we may state what kind of light-vibrations must stimulate the normal eye, in order that we may perceive it. But a moment’s reflection is sufficient to shew that those light-vibrations are not themselves what we mean by yellow. They are not what we perceive. Indeed we should never have been able to discover their existence, unless we had first been struck by the patent difference of quality between the different colours. THe most we can be entitled to say of those vibrations is that they are what corresponds in space to the yellow which we actually perceive. —Moore, P.E., p. 10