These plastic paint palletes are generally very cheap and easy to get to, but if you want a quick way to just free up some white space on them, here is an overnight tip for easily cleaning it off.
I discovered this one day when I needed to apply some glue to something with a brush, and I used my palette. First liberally apply a bunch of white glue/PVA/school glue/Elmer's Glue allover the areas you want to clean some of the paint off of.
Make sure it spreads around some and covers the main areas well enough. You want the glue thick, but not too thick. Being able to slightly see the colors through is idea. Let this dry overnight. This is a great thing to do before you go to bed.
By the next morning, all you need to do is peel off the glue layers, which will pull up most of the paint with it.
These are all the glue peels showing how much paint they took with them.