why don't you just buy a black plastic kit? You can get a complete kit from UFO for about $80-$90 that includes the shrouds, number plates, and front and back fender. I have them on my bike and it looks sick (I still have the blue frame, gas tank, and have blue handguards... along with Ride Industries graphics it looks sweet).
If you are going to paint your plastics, first prep them well by sanding out the scratches finishing with a 400 grit wet sand. Then clean them real well making sure to get off all of the 2-stroke oil, armor all, etc or else you will get "fish eyes" like crazy.
The ONLY paint I would even attempt to use is the Krylon Fusion paint. It adheres great and if you know how to paint you can get a really glossy finish out of it.
It does get scratched easily when you fall though, but it is also very easy to touch up. Just wet sand it and put another coat or two on it and ta-da! New looking ghetto plastics.
I'll be the first one to admit... the side panels on my 02 YZ250 are painted... and have been for the last four years.
I painted a bike years ago. It turned out very nice and allot more shine then new plastics. However, after a year of small crashes and scratches from trees and limbs it looked real bad. I bit the bullet and bought new plastics. The worst part was I never got the hours back I invested in preping and painting the old plastics. I would recomend against it unless your trying to pull off a cool paint job like studboy, but if you must it can turn out very well if your willing to put in the work needed. In my personal opinion I dont think it holds up near as well as colored plastic though.