We did some experimenting with pouring paint this summer and we just had to give it a try on some pumpkins!
You do have to prep the paint for pouring so I suggest just picking a handful of colors. To prep the paint we mixed 2 parts paint, 1 part glue, and a splash of water. This just what we did but ratios can be whatever you want/is working best for the paint you have. You can even just use some water if you don't have any glue.
Then I just let my daughter experiment and pour. We used mini pumpkins and she had four to decorate.