Does there seem to be a mark on their door where the blue paint has rubbed off on your door? Is it at the same height?
Try and get some evidence under your belt before you approach them.
I have a company car and back last summer some woman opended her car door against it, looked at me and carried on getting out rubbing her door against me passenger door. I wound down my window and asked her to have more respect for other people's property. She could not see she had done anything wrong and had the cheek to answer back! (B***h!) Luckily it had not done any damage after examining it.
All I can say is keep calm about it when you approach them even though you feel like 'having a go' at them. If they do not play ball go to the Police.