I've used a freelance illustration to test how I will colour my major project. I heavily rendered this image withe the line-work with the intention of using a layer of flat colours, a shadow layer and a texture layer to achieve the final look. However, I decided this one needed a highlight layer as well.
I am reasonably happy with the overall look of the final image. I will render with line work a little less and spend more time on it as it is a bit rough here. (that was also an intentional experiment as I often feel my line work can look stiff when inked an wanted to try and retain some of the energy by working looser but in this case I feel it is a little too loose.)
The shadow layer is added at 25-30% set to multiply and the shadows are applied as one colour. Setting the layer to multiplay let's me draw in shadow shapes without worrying about the colour as it picks up some of the base colour.
The highlight layer in this instance was set to overlay at 25% and painted in with pure white.
Adding the texture layer at 100% soft light I found has "unified" the whole illustration and sort of normalised the colours to a working palette. It feels like a cheat or short cut but I think it works fairly well. I would never en up at the colour choices of the final image if I had made the palette on my own.