First Prime in whatever color you feel the most comfortable with, I normally prime in Black, but the store i get my primer from was out of black, and I dont like priming in white, so i got Grey.
Use a heavy wash of Dark Flesh, or a suitable dark-brown color and hit all of the recesses in the chitin on the stealers, we are going to leave the fleshy bits for later. Repeat this a couple of times untill you get a good shading effect, some primers will make a wash of
GW paints come out flat and boring, but if you layer it on with a couple of washes it should come out ok.
Next use a thined down Bleached Bone or suitable tan-color to paint all of the chitin on the model, leaving the deepest recesses dark brown, and trying not to cover up all of the color underneith, you want to shade it with the tan, but not hide it. Drybrush the same tan color onto the back-ridges of the model.
Now use a thinned down Skull White to paint over the tan color, this part can be trickey because you dont want to make all the tan invisible, so you have to thin the paint down quite a bit, and you dont want it to flow into the recesses and spoil the Dark Brown, so you have to moderate the amount of paint of your brush very well. As with the tan from the previous step, drybrush the back-ridges.
Here comes the fun part. Now paint all the fleshy bits with Liche Purple or another suitable dark purple color, after it dries paint the same spots with a wash of a black/blue mixture, somthing that is close to black but still has a shade of blue to it. After it fully dries do a quick light drybrush over the purple with white. While you have the white out and ready to go, run a second layer over the tan like you did in the previous step. You may need to correct some of the overspill of the darker colors from the fleshy bits, thats fine because a pure-white section right next to the fleshy parts looks good

Also, paint about halfway up the claws with the pure white color. The claws themselves should be grey at the base, and white on the tips.
On to final detail!
to finish up the Chitin all you need to do is hard-line some white on the raised edges as a finall highlight. To do the claws/teeth all you need to do is use Ice Blue or another suitable light blue color on the grey sections, and blend it into the white at the tips.
then you are done! enjoi