Been meaning to try this for ages: Painting into a white undercoat while it’s still totally wet. I like how the white lightens the colours. In the top left of 170116b (above), I tried using a pale yellow-green as the undercoat, but that gives a less luminous effect. Might still be useful in some instances.
It reminds me of watercolour on white paper. I did the background this way in 170116c (below), but painted the foreground objects without a white undercoat, so they would be less washed out by comparison. As a quick exercise it was interesting.
In 170116e (below) I tried drawing into the undercoat, doing a sort of loose hatching (thinking of Ray Pettibon). Some of the coloured parts in this one remind me of felt tip pen on white paper.