I have never seen that technique of using gradient overlays for colorizing, it's very clever and the result is very impressive. An advice of mine would be that if you're ever interested in getting smooth custom color shades rather than linear or radial gradients, then you could use SAI's brush tool for the base colors and then blend the colors using the ol water tool at a low density, about 10%-15%. However, this tedious to do and requires a great deal of patience and effort, especially if you don't use a tablet.