We see no other Red Zabraks, specifically the Nightbrothers on Dathomir, throughout the Clone Wars and Star Wars. Why is Darth Maul red when other Zabraks are mostly yellow/tan?
It's just his skin tone, and it kinda seems that in the period of the Clone Wars he is part of a minority group within his species (due to him being one of the few that looks like this). The black skin on the other hand are tattoos. The decision for this is probably to make him stand out more making him more memorable compared to others of his kin.
It is kinda like asking why Mace Windu is black. Seeing most human characters in Star Wars seem to be Caucasian.