OK, there are a lot of interesting replies and I can't answer each and every one. However I think you all miss the point I was trying to make. Perhaps that's my fault for not being more clear.
Everyone agrees that we "see" colours. But that doesn't necessarily mean that they exist per se.
Colours, as we see them, are simply
interpretations of particular wavelengths of light by our eyes and brains.
Let me put it another way. If life on Earth did not exist, and there arose another form of life that perceived the world through sound-waves (like bats), they would be ignormant of such characteristics; colour to them would not exist. In other words, the existence of light is dependent upon its observation (ie, its interpretation) by conscious beings.
What I'm getting at is the link between colours existence and observation. If noone can see in colour, does that mean it's still there? And, as we know for a fact, that certain people see the exact same wavelength
differently, then we know it is not a fundamental characteristic. There are brain injuries that can cause complete colour blindness. That is, the victim can see only in black and white. If our brains had evolved as such, none of us would see colour. Would we still be having this argument?
I think the fact that different people see different colours for the exact same wavelength proves the issue that colour are not inherent, but interpretive. And therefore, as they are ways that our brains "see" the world, they do not exist outside our brain.
I guess my question is similar to Nagel's famous paper "What is it like to be a bat?" (
http://members.aol.com/NeoNoetics/Nagel_Bat.html).
Mr Mephisto