"There's something wrong about this.. in the sun glass part. you can't do that kind of color synchronization in paint, it is so radiant. well, that's only my opinion. but if you really did that on paint, that's a really great job.^_^"
It's actually quite simple, start off with the darkest color and work your way up to lighter colors. The key is to use color that are close to eachother, not going from an extreme dark color to a medium color, then you see the big color gap.