[It's Just Eric]

For flexible parts, the paint is the same as the rest of the car, Although the preperation and primer will vary

Many bumpers come with primer from the factory. although you COULD shoot them just like they come, I find the finish is smoother when you wet sand them with 600 first
Some bumpers come in raw, meaning theres no primer. This will likely be the case for most aftermarket parts, both urethane and in some instances fiberglass.The problem with raw finishes is that to get the part out of the mold its made in, they use slippey chemicles as a mold release. To rid the bumper of theese, you need special cleaners. It also helps to flambe the part after cleaning to burn off any residue
A special note on carbon fiber and fiberglass parts...Most of them come gelcoated, in which case it will be necicarry to wet sand them with 600 grit to give you nice smooth surface to paint
as for primer on flexible parts, most major automotive refinishing companies will offer special primers. Some will also offer adhesion promotors that are sprayed on prior to primer (There are adhesion promoters for paint as well)