Exterior plastic and rubber fail by UV oxidation — the plasticiser migrates out, the surface chalks, and black becomes grey. Two approaches:
A dressing replaces the oil and looks good immediately. It washes off, so this is a monthly job.
A trim coating bonds a silica layer that holds the restored colour for a year or more, but it will not restore trim that has already chalked badly. Restore first, then coat.
Do not put a paint coating on trim. It cures glossy on a surface designed to be matte and there is no easy way back.