I really enjoy doing quick color exercises.Some of these were really simple to make, like the one ion the top left, while others were "massaged" for a much longer time, like the bottom left.
I basically start with a very simple composition choosing two colors, one that will provide the main atmosphere and another one that will contrast it. Once I have these I start adding other colors that inform not just the composition further, but also nudge the mood as needed (sometimes I go by the seat of my pants). I hope to be uploading some more of these again sometime soon (I hope).