What to do with leftover paint?

I was painting (as in art), and I still have some paint leftover. I don't have a cover to put over my palette, and I don't want to waste the paint (especially since it's a color I had to mix with another color. I don't want to lose that shade).

Will plastic wrap work?
It's just for over night. I have to stop painting for now, because I'm too tired.