overspray .If you put the parts on the car the body shop will have to remove them to paint the edges (the right way) or will Paint them on the car leaving more of a chance for overspray and it will be harder for them to prep the edges (more chance of poor adhesion (pealing )around the edges) If you will be painting the car complete at a later date just bring the parts to be painted and the car to the body shop so the shop can verify the color ,and pick up the painted parts when they are done .You will save the removal and replacemant labor and the parts will be preped and painted complete if you choose a good shop . It would be best to check that the new/used parts you will be using fit corectly before having them painted !!!