This is the start of colouring. There's a bit of a leap between this image and the completed one. I like to keep a lot of my elements separate. With medical stuff, you never know if sometime in the future you'll want the same image, minus the muscles and the ligaments, or you never know if you'll want to see through something. Often it's anticipating what my clients might want. So just about everything is it's own layer group: each individual bone, each muscle and tendon, shadows and highlight etc.