Last year at a Crawfish Boil, I looked over them as we picked. There were a few variations, but 1 color dominated the bunch, with a Green-based color in 2nd Place. That dominant color is a major focus when I'm shooting Natural-colored Craw imitations. Black/Red was the 2nd most abundant because there was alot of Belle River seeded Crawfish there. Pond Crawfish are usually Green or Brown, usually smooth color. Deep water and Dirty water Craws can tend to be White-ish and Tan.
Of course, their color kind of changes through the Seasons, so have a few options of Natural Colors.