You shouldn't have any differences in paint colour wether it be a metal panel or a plastic bumper. If the jobs done 100% correctly it should be a match no matter what panel or light condition.
Actually I would have to say that is not entirely true and this can easily been seen on silver coloured car.
From factory the paint on the plastic panels can look ever so slightly different from the metal panels - I witnessed this on my last golf which was reflex silver and you only noticed it if you looked at the plastic panels from a certain angle in a certain light. It was ever so noticeable on the petrol cap cover.
I have a good friend who has his own business and has been respraying cars for over 30 years and his work is second to none. He will always tell his customers that painting on plastic parts can result (doesn't always) but can result in an ever so slight difference in the colour despite having an exact 100% colour match with the paints/materials used.