Maybe what I see as red is actually what I see as blue to someone else.

This is a very common interesting thought, but what I’ve started thinking is even more interesting is this related thought:

Why does red look like it does, to you? I’m not concerned with how other people see red here, I’m just thinking about a single person (me or yourself, for instance). Why does red look like that? Why not differently? Something inside your eyes or your brain must be deciding that.

You could say “oh it’s because red is this and that wavelength” but what decides that exactly that wavelength looks like that (red)? There must be some physical process that at some point makes the qualia that is red - but how does it do that? The qualia that is red seems to be entirely arbitrary and decidedly not a physical thing. It is just a sensation, an experience, a qualia. But your eyes/brain somehow decides that ~650 nm wavelength translates to exactly that qualia. What decides that and how?