help need with painting question

[passat turbo]

hi i hope some one can help im at collage doing a restoration course so far ive smoothed my rear bumper of the strip nd so t looks like a one piece bumper rather than a 2 piece we are doing the side skirts at the mo which is no promblem but the bit were stuck on is the bits on the bottom of the doors i have removed these and there half hard plastic half rubber type what i was wondering is how have ppl colour coded theres and made sure the paint sticks to the door bottom bits last thing i want is to paint them all up and have it flake off really quickly so any help would be great thanks