Ataraxis, if the brain makes you think you saw another colour, didn't you then see another colour? The impression is more important than what happened.

Also, what if we actually see another colour all the time, but it's so weak that we don't realise it? It is overlayed by all the other colours. It's a silly thought, but what if our brain could take this colour and enhance the vividness of it? It's very unlikely, but possible.