The way I paint bone is I cover the whole area with plain ole bleached bone, then I make a wash using thinned Dark Flesh. Maybe around 25% paint and 75% water. Also if you have an art store nearby pick up some matte medium, they should have it where they sell the acrylic paints.. mix some of that in with the water, i've found it helps the paint stay in the cracks. After that's done I do a very light drybrush of bleached bone to cover up any streaks that might be there and to pick out highlights, then I'll do another even lighter drybrush of skull white. I noticed with the matte medium wash it gives the bone sort of a "dipped" look if you don't drybrush it after.. so you can get those kind of results but with the wash you'll have more control and you can also apply several coats too if you want it to look darker in some spots. It may seem complicated at first but once you do a couple of models like this it's easy and you won't want to go back

Here's a pic of some of my chaos marines that I did with this technique