I think I said that recently I draw many practice works, not good enough or not even being a whole picture. Some of them were for Derwent Academy classes (derwentacademy.co.uk) and this is one of the lessons and I really like both technique and the way the pic turned.
Obviously since it was class homework, not much credit goes to me, but I still like it and maybe some, who see it, will like it too or the technique ^^
Doing that is pretty simple You put light colour under, blend it and cover it softly with dark one, then scratch the dark layer off. It's nicely explained in the lesson video, so if You like You can go through that little course there(- it's free and they giving You pencils if You finish- I'll tell You in about two weeks if the box arrive), or You can find other tutorial on that technique on the Youtube, or here. ^^