Skeletons are slow to do if you want them to look good. Sure, you can drybrush them and do a reasonable job (and I have 16 Warhammer skeletons with which I did just that), but as this is my only mini of this kind I wanted it to look a little better, so after priming it with white paint, I painted it with black, then carefully painted in the colored parts of the bone that I wanted to show up (as opposed to the black parts, which would be the gaps between the bones, but I'm sure you figured that out). The color I used was Snakebite Leather mixed with a little Skull White. I did several layers over that, each with more Skull White and less Snakebite Leather, until the final highlights were done in nearly pure Skull White. I then went over the gaps between bones with black ink to make sure they were distinct enough to show separation from the actual bones (particularly in the ribs).
The metal bits binding the "broken" part of the troll skeleton together are in Chainmail paint, as is the blade of the greataxe. The haft is Bestial Brown. The eye sockets have a tiny dab of Scorpion Green just because I think it makes them look more spooky than just an empty black socket.