Draw your current mood as an abstract landscape using only three colors.
Three colors is the right number. Two is too few to build a landscape that reads. Four starts to let you off the hook — you can paint anything with four. Three forces you to assign each color a job: ground, sky, mood. You will end up making real compositional decisions instead of just picking what looks nice.
The "abstract" part is doing important work too. You are not painting the field outside your window. You are painting the texture of how you feel right now. If you feel scattered, the landscape should look scattered. If you feel patient, the landscape should be patient. The prompt is a mood translator, and the constraint is what makes the translation honest.