I realize every artist has a different working process, but if you're anything like me you sometimes will have leftover paint - acrylic or oil/cwm! So, over time, I've worked out a few solutions:
1) If working in acrylic, I highly recommend you watch this YouTube video which gives you all the info you need to keep your acrylics wet for days: YouTube Video - Keep Acrylic Paints Wet 12+ Hours!
If you want to use the cafeteria tray and tracing paper I use, here are the links where I get the right size tracing paper to fit a nice large cafeteria tray:
14x17in Cafeteria Tray: https://amzn.to/2RYFkId
14x17in Tracing Paper: https://amzn.to/2SgL8xm
(Don't forget the blue shop towels!)
OR - have a gessoed scrap (or "slop"!) panel ready and waiting for any leftover paint if there isn't enough to save! You may find your scrap boards to turn out amazingly well as you keep adding paint (without thinking) - develop it as any other painting and see what happens!
2) If you are working in oil/cwm and have leftover paint, here are some solutions:
I have adapted a Masterson Sta-wet Palette to work for oil/cwm. I lay a sheet of freezer paper (cut to the size of the palette) in the bottom, place leftover oil/cwm paint on the paper, add a few drops of Gamblin Gamsol, and then cover with saran wrap. Close the lid tightly and this should really keep for days and up to a week. You can keep checking every 2 days, lift the plastic wrap, and place a few more drops of Gamblin Gamsol on each paint mound if needed.
To keep cwm (with Gamblin Galkyd Gel mixed in) nice and fresh, I actually purchase these candle tins with lids; this keeps the cwm/GGel moist for at least a week.
OR - have a gessoed scrap (or "slop" board!) ready and waiting for any leftover paint if there isn't enough to save! You may find your scrap boards to turn out amazingly well as you keep adding paint (without thinking) - develop it as any other painting and see what happens!