I hate wasting paint. For that matter, I hate wasting anything! So when I'm done with  my palette paper or paper towels and there is still paint on them, I"ll start pushing the paint around to see what develops. If it's interesting, I may pick up a sharpie to complete the image. Or I may add additional scraps like used post-it notes or scraps of paper and other stuff lying around my studio that I couldn't bear to throw away because they could come in handy sometime. Here are a few samples of my abstract art made from studio scraps:

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