What makes some weeksii green?

[Hybridfish7]

I see pictures like these, with the green heads and all that, but then I see every other weeksii on here without the green. Is there anything that makes them have the green or is it a locale thing?

 

[Krismo962]

black substrate

 

[Burbotman]

black substrate

This right here. Black really brings out the greens

 

[Hao]

It all comes down to genetics. Some weeksii can turn really green on red garnet/black sand. While some maybe not have any green at all.