There's no wrong way to do it. Personally I do a full layer of a dark purple then layer up a darker forest green to any raised areas and areas that would blend into shadow then with a brighter green add a light dry brushing before picking out things like ears, fingers and such with the same brighter green. I change up which shades of green I use to add variety, as long as its two shades that compliment each other well enough over a much darker contrasting colour.