How do i get negative colors?

[Zondac]

I was playing in this zombie server, then i saw that you could make the colors go inverted as a zombie, and i was wondering, how did they do that? How do i get that post processing-effect? I'd be very happy if someone could help me find out how

[Hakita]

Lua.

[Zondac]

[QUOTE=Hakita;20268845]Lua.[/QUOTE]

kthxbai

[Whiterfire]

ctrl-i