CA18 red top? CA 18 green top?

I'm buying a new engine for my S13 chassis 240sx, and I was shopping for a CA18, and have seen that in some pictures and places, they're listed as redtops, and in others, they aren't listed as redtops, but the valve cover is green.

Whats the deal, whats the difference, if there is any?


Which is better?