2006 Ford Mustang Base Coupe Rocker Arm Molding L & R Side

I have a black 2006 Mustang Base Coupe:I just had my Mustang repainted
the same color "black" great paint job except for i noticed when i went to pick
it up the rocker arms didn't get painted when i asked the shop Manager why
didnt the rocker arms get painted as well? I was told  my rocker arms are
made out of a material that won't hold paint and they would chip in time it
wasn't the right surface that could be painted on because of what there
made out of? I also just had a new chin lip put on my front bumper and they
told me the same thing with that part it wasn't paintable?  Are there different
types of rocker arm moldings installed "stock" that can not be painted
because of the material used to make them? Just wondering what do they
use with the cars that are painted red blue orange ect..? Are they installed
with a different type of rocker arm that can be painted on? If so where do i
find this type od rocker arm because my have faded out so much they look
almost grey and they look extremely old now on my newly repainted
Mustang? They chin lip isn't that big of a deal its black already but it seems
strange to sell something for the exterior of your car and they make it with
material thats not paintable? But the stock Mustang rocker arms are my main
concern? Please some feedback some comment on this? HELP!!