I think Mary used Krylon. I used Valspar for plastic. Even tho I sanded to rough up the surface, it still comes off if it gets scratched. I think as long as you buy a paint for plastic you'll be fine. They make a plastic primer, and maybe I should try that next time.
I just picked up 2 more kittly litter buckets from another lady on freecycle. They are different. The lids don't come off, but hinge at the top. I might like that, too. I think I'll make them into panniers and then decide on my favorite pair.