bf1104:
However different people see colours differently too
Well, in so far: it's unknown whether what I experience as "red" is the same experience for you. We just don't know, and probably can never know. Also how we experience a colour is determined by the colours around it in an image, but that still doesn't change the actual colour.
However we all do agree on what colour is "red", and what is "green". We will also most likely agree on which two reds are the same, an which are just a bit off, though the threshold may be a little different.