Colour does exist. 'Plasmic light-rays', 'photons', and 'radiation' may or may not exist, and they may or may not cause my seeing colour. But colour does exist with or without them.
Colour refers to qualia*. I know that there is red, blue and even green. I can see them in my head. I am more certain that there are these colours than I am sure of the existence of this computer (Think Descartes' Demon, or if you must, The Matrix). I might not be sitting at a table typing on a keyboard. There may be nothing 'white' in front of me. But there most certainly is some white here. I can see it. I might be imagining that it's 18 inches in front of my face, but I don't see how I could be imagining that I have an experience of white.
And colour does indeed refer to your experiences. You can have a colour in your imagination, while at the same time having no 'electromagnetic radiation' of the type to stimulate your 'rods and cones' shuffling around in there.
We assume that, while they have different causes, the colours in our heads and in the outer world are the same. We could be wrong. Maybe they are completely different things. If the title of this node means only that Outside-of-your-head-colour doesn't exist, I will not argue. But by the good solid definition of colour as qualia, colour exists.
* Which is not to say that it doesn't refer to something else too, but it does indeed refer to qualia, and qualia surely exist.