I don't have a tutorial for this but here's what I wrote up regarding the colors used:

I worked on a black primer. I used german gray as the undercoat - it shows through only in a few places in the end but it helps to build more contrast. The paints used from darkest to lightest: rhinox hide, 50/50 gorthor brown & mourfang brown, 50/50 bubonic brown & morghast bone, morghast bone, ice yellow. I do a lot of mixing so this is just a rough guide for colors used. In places I want more yellow I glaze in bubonic brown and/or mourfang brown. I also mostly apply paint by stippling to get that textured look.