New way of painting. Start with ambient occlusion, because a ton of your values and shadows are gonna come from that anyway.
Sculpt by Bruno Camara
There are two kinds of gradients, one that moves toward you and one that moves away from you. You need a separate brush for both.
Be careful when it comes to subtle value differences. As soon as you enter that territory, your mistakes will cost a lot of time to fix. Make sure your big shapes and values are placed correctly before that happens!!