I've been told the plastic tubing doesn't hold paint very well, so it comes off really easily when hooping. I wanted to do some cool hand painted hoops until that I learned it doesn't work so great =/ I have seen a page where a girl did some really cool tie die paint jobs by blowing paint around inside her polypro hoops with a compressor, but of course you could only do that with translucent tubing.
I think there are a few threads on this topic already that could help you out.
Here are some pictures of Beth Piver's spray painted hoop:
it scratches off unless you put clear vinyl over it. its also very slick.
There are plastic-specific paints for lawn chairs and such. The ones in the Krylon Fusion line look good - lots of colors and two metallics. I would imagine they'd stay on pretty well since they are meant for lawn furniture. http://www.krylon.com/products/fusion_for_plastic/
Spray painting would be a fun project. Could be frustrating if you are trying to get a perfectly even coating for a single color hoop, but the possibility of stencils and artsy designs is intriguing.
I'd love to find this line in Japan but I don't think there is a distributor here. I'll have to look for some domestic products.