Red? Like an American Indian skin tone? That would've backfired, I think. If anything, it would play up the "devil" appearance a bit much, at least on color television. And then on B&W (black and white) sets, he'd have looked more ethnic.
Funny to think about that, the idea of color television being new and B&W sets still predominant. My parents bought their first color television in 1970, while some friends of theirs got one in 1968. Back in the time of TOS, a majority of people were watching it on B&W sets (based on known statistics
of household color TV ownership). Just look at the stats on TV sales
. It wasn't until 1972 that color TV sales surpassed B&W. Strangely, after 1974, B&W TV sales began to climb again, probably due to the economic strains of the day.
So, I'll have to bet some of the choices for color had to be made to look OK on both color and B&W TV's. This might explain some cases when things looked a little odd, color-wise.