How do I change the colors?

I want to make the colors Negative/Inverted. How am I supposed to do that?

r = 255-r
g = 255-g
b = 255-b

Thank you very much, but where in the script should I put that? (I don’t know anything about color modification)

Do you mean you want to change the colors displayed on the screen? I think there’s a post processing effect for that.

Yes, I do mean the colors displayed on the screen.
I have looked for a post processing effect, but i couldn’t find any.
I also want it to be hot-buttonable, soooooooooo…