Rust-O-leum has a paint for plastic and Home Depot's house brand does too, and I think the paint guy told me they had a third brand of paint for plastic too. I know he told me that he uses the house brand instead of Rust-o-leum because it wo0rks just a well and is significantly less expensive.
Regardless of the brand UNLESS IT SPECIFICALLY says it is for painting plastic, it will probably peel and flake off -- even epoxy won't adhere to the soft plastics correctly. They need special solvents and also need ot remains flexible like the plastic they are applied to.
My statement about fumes is this simple -- in my experience the fumes come from the paint itself not the propellants, especially with the new green house gas concerns. That means if you brush it you still have the same volatile fumes from within the paint itself to deal with. Initially, spraying even with an air compressor, you will have not only the fumes from the paint to deal with but the spray droplets too, but the droplets soon settle out -- that is where overspray comes from.
Probably the cheapest and easiest way to make the bucket light proof is to wrap them in heavy black plastic and secure it with duct tape. That way you can patch any tears easily and it is easy to remove and start over if you want to. Black duct tape is good to if you have the patience to make sure it is all tightly adhered to the bucket, even in the little spaces between the ribs at the top.
Personally, I think that some people go over board with the light proofing. I've read comments from experienced Hydro growers here on the forum that say they never had any problem using all of the colored buckets -- white to black -- without light proofing them. Just do the best that you can and you should be okay.
Quite possibly you don't even have to worry about any thing except the clear plastics like are frequently used in smaller totes. I remember, my mother used to root cuttings from all kinds of different plants in clear glass fruit jars and water sitting in the window sill and she never had any prpblem with the light killing roots.
You pays your money and takes your chances, Popeye. Just use what ever you want to use and see what happens. I do know though that I've never heard of any problems in soil or soilless mixes like Black Gold, coco, and so on.
HAPPY NEW YEARS and GREAT SMOKING.