my edit is crap, you need to study the character and work out exactly how your gonna go about it, work out a sensible palette, the original reference has 2 shades but I'd probably personally use more to make that transition less drastic.
my edit only took a few seconds to make and I only used your existing colours. As im9today said there should be much more contrast, the lighter shade should be lighter and the darker shade should be darker..
heres a slight suggestion.
