While I'm standing nose-to-artwork painting little details, I try to avoid having a big clumsy palette in my way. I'll grab one of the little canvas or masonite panels I use for comps and mix paints on that. When I go to another section of the painting—or even to another painting altogether—I usually just coat the board with colored gesso and keep using it.
When I see an accidental shape on the "palette" that intrigues me, I set aside the board and grab a fresh one. Once the painting I’m working on is finished, I'll go back to explore that suggestive shape with the leftover paints and try to coax out whatever it was that caught my imagination.
The "bonus" image below is an example of the small boards and scrap I use as working palettes. The one at bottom center with “Fredrix” on it is this week’s image, turned over to show the back side. This was one of the palettes I used while working on my cover painting for Brandon Sanderson's WORDS OF RADIANCE.