What shade of green should I use to paint mirialans?

Have you tried GW's Elysian Green? It seems pretty close.

I'm not sure there's a "right answer," as skin tones seem to vary quite a bit. Barriss Offee, Master Luminara Unduli, and the Seventh Sister all tend to the yellow end of the color spectrum, with Luminara being somewhat more greenish. As a playable race in the MMO, SW:TOR, there are a ton of (non-canon) options for skin tone, hair color, etc.

I'd probably start with some sort of olive, and tone it up or down from one individual to the next.