We read the book, "The Very Ugly Bug" by Liz Pichon. It is a very cute book about an ugly bug that is unlike all the other bugs and wishes she were prettier. She tries to change herself only to find in the end she is perfect the way she is.
Set out paint, brushes, and paper or object to be painted before you invite them to the table.
I use plates for the paint that I can either dispose of easily or wash off later. Plates seem to make it easier for them to dip their brush without making splashes or color mix.
Have wet and dry towels close by.
It would be nice to have them wear a smock, apron, or old shirt. All of my kids have hated any of these. No big deal. I just use washable paint for now and throw a little oxiclean in the laundry.
Cover your work area. I use a big piece of oil clothe. It's easy to wipe up after.
Have a "busy, but not messy" project to hand them as you do a precursor clean up and wiping of hands-like crayons and paper.
Have a simple snack prepared. They can go eat it while you finish the big clean-up.