No such thing as a "UKARA license."
Painting a two tone and turning it into a RIF is not illegal for a 14 year old per se, but if he has done so without being someone who plays at a proper airsoft site then he has broken the law and has no defence against prosecution. More to the point: who's going to know?
In relation to him taking it to sites, theoretically nobody should even ask him why he has a RIF; it's not illegal for him to own it after all. But if someone does happen to ask and he tells them what he has done they might decide he is a dick. If it's someone who runs the site they might think that he's not only a dick but a liability, and then they may choose to not let him play, as is their right.