It's interesting that the dark purple skin with red markings has a completely different back pattern to the other three in-game ones. Unfortunately it means you can't really mix it up with the other skins because the patterns overlap and look kinda messy and weird. But I can swap the colour channels around within the same skin and get some interesting results:
This is also the only skin that has brightly-coloured legs, so when you swap the colour channels around you get even more colourful legs. But they probably aren't any worse than the original.
The rest of the colours here are generated from one or more of the other three skins (mostly using channels from all three skins at once).
First some really striking shadowed ones. The first purple one is my favourite by far:
A sunburnt one:
And a range of other bright colours that have never seen the light of Azeroth:
I hadn't realized the legs were different too; that explains a lot! I did notice the eyes, however. 
Yes, that's the one I meant. 
Have fun! 
