It's a mess...but oh what FUN!
I used to only let the girls open 1-2 colors of paint at a time, to keep the messin' and mixin' down. Then I got the idea to put a little bit of each color in an egg slot. Then I don't have to be always bugging about putting lids back on. When they're done...it's easier for them to help with clean up. They just put the old egg carton in the trash! The mat covering the table under their artwork is one of those 'splat mats' you use under a high chair. They wash off very easily. An old plastic tablecloth works well, too, and you can just throw it in the washer. When the girls are doing messy play (play doh or paint) on the floor, we use my mamma's old trick.....an old cardboard fabric-cutting board. Just shake it off outside and fold it up! You don't even have to wipe it off - the dried paint is just a memory of many fun hours.