Using layers of Overlay light
Here’s a technique we’ve been applying recently to achieve a watercolour-like lighting effect. Create several new layers over a painting or coloured line art. Three or four layers should do. Apply the Overlay layer effect to each layer, and perhaps set one to Hard Light if you want a very chromatic highlight. Then apply alternating splashes of warm and cool high key colours on to each layer. This creates a multi-hued splash of light.