There is an ongoing struggle that all watercolorists must face when they begin using watercolors. To clean or not to clean their palette.
Watercolor paints, especially good ones, are not cheap. If you get a good mix of colors, you want to use every last drop.
On the other hand, it's easy to lose track of what colors are what. What colors, exactly, did you put into that mix? Where can you put down fresh color without it getting messed up? How do you keep your pure colors from getting contaminated?
Many seasoned painters don't worry. They wipe off the contaminated areas or live with the contamination. They develop a system so they know what paint is where and they never clean off their palette. Other masters are exactly the opposite. They clean off their palette after every painting, so they approach each one fresh and they have complete control over the purity of their paints.
I suspect neither system is better than the other. It's just part of the personal style each artist develops and each artist must learn for themselves how best to manage their palette.
Surprisingly, I, the messiest person I know in this world, am falling on the side of the clean palette. I want to start each painting with fresh color and a clean (if unavoidably stained) palette. On the other hand, it kills me to wash away perfectly good paint. So what to do?
I've been using the left over paint to practice washes and make charts between paintings, but when I don't want to actually work at it, I've been doing WATERDOODLES! Easy, and it's fun. Plus you learn brush control. That's my story, anyway, and I'm sticking with it!
I think the photo is self-explanatory.
I use cheap watercolor paper, and paint on both sides. I may stop that though, because it might be fun to cut these out and use them in my art journal.