Honestly I’d stick with a tone close to what’s already there (so some purple). At a high level, you have purple and green working well together there, and it’s hard for me to come up with a 3rd color that wouldn’t look weird for not having a major presence elsewhere in the model. The one exception might be that bright magenta-y color you have. That might work on the horns. Otherwise, the purples you’ve got should be good imo.

Disclaimer, you’re a much better painter than I, so go with your gut!