I use black now, since my morphs are all yellow based (pastel, spider, and albino). I once used DARK blue, and it looked fairly good, but the black is best for them. I would say to use white on the darker morphs for good contrast.
If you want to use colors, use the opposite of the snake's main color - yellow snakes should be on dark purple to black, Reddish brown should be on turquoise (believe it or not) to bring out the red. My partner is very good at color wheels and combining and contrasting.![]()