Yellow and green carry almost no load at all. 18 AWG or so will be just fine. Regular "hook up" wire from the electronics stores will work.
Purple - not a whole lot of load, but either match what comes with the monitor or go one size bigger.
Brown - Go big. 8 AWG or so.
Orange - I'd go one size larger, 8 AWG rather than 10 just to be conservative.
The following FAQ gives a guide to how to pick wire gauges and why.