Introducing colours using Multiply and Overlay modes

To add colour, I group my greyscale layers and create a Gradient Map using two points of colour: a very dark brown (almost black) and white. I clip this to the layer group by holding Alt and clicking just in between my Layer Group and the Gradient Map in the Layers panel. This gives the tonal drawing some warmth in the dark areas. Then I paint into the light areas of the skin with a light yellow on a layer set to Overlay. This will add colour while preserving the underdrawing.