I start them off as a really tiny blob-pose in colors and then blow it up to size, refine the edges so I can tell what they are, then I use the background texture to my advantage and duplicate my 'flattened' drawing/colors layer like...4-8 times. 1st one is set to multiply, second to 'soft light', 3rd one to about 15% opacity. This establishes my darkest darks and I proceed from there to set all my OTHER layers to various opacities (the top being the future lightest so it's at 100%) and use a layer mask to blot out everything visible and 'paint' in the lights of that layer with white so they're visible in increasingly smaller amounts showing through. Then once I have all THOSE values established, refining any roughness for a bit.