If I spray paint them, I'm worried that the paint will chip off and look like crap. I was looking for some type of alternative like the possibility of a powder coat or something similar.
As they are PLASTIC, powdercoat won't work. It uses electrolisis (sp?-> it's late) to bond color particles to metal, which is then baked to melt it on and make it last.
Plastic can either be melted and a color element added and recast or be surface coated with something. Basically, paint is about the only cheap way I know of to recolor them. Anything else I know of but vinyl will cost more than alloys (400-700 for decent cheap ones).
I'm not sure if vinyl would good or not. Alot depends on your intended look, finances, and resources.
I like the naked steelie, as detailed in my steelie centercap thread . . .