that's really nice!!
you look like your doing well thining your paints, which is one of the hardest things to learn at first I think
Re: the primer, a bone colour is fine especially since it'll make painting the bone bits easier along the spine, I should think.
I tend to use black myself, because I like my things to be quite darker, and they tend to have a lot of metals. I've done white and it makes the whole miniature really bright overall, which is fine if you like that or you're painting a lot of light colours over it.
Something you could do with the bone bits specifically, if you wanted, is paint them a nice solid brown, then use your ushabti bone almost all the way down to the base of the bone area. You might want two coats of the ushabti this way though. I usually will do a zandri dust base coat first, over my darker primer, then ushabti, then finally an off white highlight. But that's just the way I tend to paint bone, if you're happy with yours no worries =D
For washes, I don't know nids well so just looking at the purple I might go ahead with a druichi violet (the purplish wash) or maybe a nuln oil (basically black) to darken it first, then go back over with your main colour, carefully, before you highlight.
For bone washes, I tend to use either sepia brown, or agrax earthshade.
Anyway just my two cents