We only see what we need. So until evolution decides it's a good thing to be able to see gamma rays not much we can do there and not much point. Would be interesting if a thousand generations down everyone has synethesia.
I agree with you, my point was that we would be blind from seeing too much (radio waves e.t.c.) The color thing is a whole different matter, to see new colors we would need entirely new eyes that work differently and perceive things differently.