if you want it to last longer use duplicolor auto paint. i had to do this with my Treo 700's Extended battery cover. the cover that seidio used didn't even come close to matching.
so i went to a auto parts store and got the closest matching paint.
i beat the hell out of that phone/cover and it barely wore after 6-8 months.
if you wanted it to last even longer after you spray it, use a clear coat on it and just clear coat it every few months.