If you find them in the right mood, just about anything will work, but the aforementioned colors seem to be the best producers. I recall one time when I lost all the flies I had tied for chums (like Todd said, bring bunch, and then bring more) and all I had left was trout flies. Turns out a black woolly bugger looks just as good to bitey chums as anything else (or at least it did on that day).

Have fun!