Sometimes it’s a blank piece of paper, sometimes paper I have used to wipe off excess paint or ink:
I will use my cheap box of watercolors, some left over bits of colored paper and add some details with markers.
Messing around like this is great. There are no expectations, no right or wrong, no mistakes. Just cheap bits of recycled paper and cheap materials, so I don't have to feel bad about wasting money or materials. And the best thing is that without all that pressure I feel free to experiment. With sometimes surprising results!