Yikes They look like one wrecking ball pair up, smexy and likely built harder and tougher than actual rubies. Could only imagine what it would be like to be on both their bad sides at the same time. Great work on making them look both alluring as well as deadly. Good luck managing to put a dent in those devilish abs
An easy way to contrast color is; creating a layer (Layer B) abover the one with the base color ( "layer A"). Set the "Blend Mode" of Layer B to multiply and apply the same color of layer A to layer B. It's a good way to get a darker color...
...or are you talking about giving it some "shine"? D:
Sorry if i had told you things you already know xD
I like how you are experimenting with different coloring techniques. I'm not familiar with these characters, but they remind me a bit of the Puma sisters from Dominion: Tank Police, although they were catgirls not devils!