The rear filler pieces are argent to match the tail panel. I believe Rodger Gibson sells the correct argent that you can spray out of a gun but it's very expensive. A lot of people like to use the spray can argents. Totally Auto sells argent in a spray can but the color is not correct for the 73 Challenger tail panel and filler pieces. They sell a light and a dark argent. In order to get a color that is close to the factory finish, my painter sprayed the dark argent first...while it was still wet, he sprayed the light argent ontop of it. That resulted in a color in between the light and the dark.
Another option is to look for Krylon Fusion paint. I havn't tried it but people say there is a color in that line of paint that is very close to origonal. No spray can paint is going to have the same amount of texture as the factory paint. It just depends on how correct you want to get and how much money you want to spend on the paint.
As far as the front fillers are concerned. They aren't painted at all. They are just the natural grey of the rubber. Here is a picture that shows the front filler on my 73.
The front fillers strips clean up very well with just a light wet sanding. It depends on their condition of coarse. Mine were in good shape. They were just dirty. Here is a picture of how the center filler turned out. It looks brand new. The two end filler strips cleaned up just as well.