Well to begin with, don't ever use Black or white. They don't exist ! :) Keep in mind that handpaint is about faking volume. if you take a piece of wood stack it with another, the crack in between isn't going to be black, it's going to be SLIGHTLY dark than the original color.
From what I can see, the light direction for your highlights is coming from both above and under. Decide where you want your imaginary light to be originating from and keep it consistent throughout the texture. Quick example of light direction I found in the handpainted thread: You can see it's coming from the upper right.…