Spray paint. Two -three coats.
I'm sending you several videos because I don't know if you want to do "professional" painting or just paint with any kind of paint for a kids sandbox etc...
Probably the best paint for this job would be a latex with a primer underneath. An enamel would likely crack and peel from any oil residue that might stay on the rubber before painting. Wipe the tire with a vinegar, solvent solution to remove any old residue.
Hi, this looks like a beautiful method with spray paints
Wash with lots of soap & hot water.Dry.
Prime it two coats, and paint.
Thanks very much