Black like the game insists.
Boys = green, girls = red, just like their parents.
Boys = red, girls = green, opposite of their parents.
Flip a coin for each kid: heads = green, tails = red.
What's wrong with mixing red + green = brown?
Blue! A throwback to Ray's dad!
So I did the flip-a-coin method, which is how Emerald and Summer got green hair and Brent got a rusty-red color. As "Blue" came in second, I feel OK in using that when and if the mood strikes me. As soon as Alan hits Child stage then I'll do the same for him, and so on.
At least with the Serlings, they're all blondy-blonds, so there's no effort involved in hair color choices. ;)