You don’t have to add colour to all of a painting or watercolour sketch. With these two portraits I made marks where the shadows were and left other areas blank. The result still gave shape to the image but allows you to do something quickly (ten minutes). Using a limited palette also makes it easier. If you are not mixing a lot of colours you can create some interesting effects in a fast way.
This is college work, made for a daily sketchbook of observational drawings and paintings.