After some feedback I've taken the previous design, as well as a new tweaked one, and tried out a few colour schemes based around a 'duller' version of the main villain:
Colour-wise, I think 7, 8 and 5 work quite well. I think his bulk being purple with a few coloured highlights works but I also quite like the green chest with purple everything else.
I've also got a third design for his chest, here are all three together:
|This isn't the final colour scheme, it's just helping to see which design is most successful.|
I like designs 2 and 3 the most. He still matches the lead villain but 2 distinguishes himself a little more from him than 3 does. Any feedback is much appreciated! :)