A blind person could say "There are no colours that I can see". That doesn't mean there aren't any colours.
in a way it does. in a very real sense colors aren't colors until they're seen.
I don't think it is coherent, in any sense, to say that
x is not identical with
x unless
P; surely the proposition that everything is identical with itself is unconditionally true. Perhaps what you meant to say is that there are various wavelengths of light and there are sensory perceptions of these wavelengths, these are distinct phenomena, and only the latter can properly be described as 'colours'. I think this view is relatively uncontroversial, but it is important to note that on this view, upon being perceived the wavelengths don't
become colours, the colours are the perceptions themselves, which occur in the mind of the organism that perceives the wavelength. If this is accepted, then it would be appropriate to say that colours might not exist if there were no organisms which could perceive wavelengths. (I say
might because we have not established whether or not colours can be exemplified in any other way.) However, it does not, in any way, license one to say that the existence of blind persons legislates the ontological accuracy of their subjective assertion that there are no colours, though it may serve as an epistemic justification for such an assertion. The much weaker proposition which might be justified is the claim that there are no colours in the minds of blind people, or, more conservatively, there are no colours in the minds of blind people which exist by virtue of them having perceived wavelengths of light.
I hope that I have not misrepresented what you meant to say, I did try to interpret you as charitably as possible, but if I have misconstrued what you meant then I would appreciate clarification. Though, I will anticipate the response that they (light wavelengths and colours) are the same thing identified by different properties, this is a perfectly reasonable view, but it doesn't justify claiming that there is a sense in which there are two distinct objects. Such a line of argument would commit one to the view that there is a sense in which the same man is not identical to himself if he is identified by his voice instead of his physical appearance.