urban legend ?
No urban legend. The LW had a squadron specialized in Allied planes. They got their planes from captures and salvageble crashes. Same thing for their spare parts. I have seen pictures of B-17s with LW colours.
Although, I never heard of fighters weighting in a fight, I have read reports of bombers joining the bomber stream to provide data to the German ground control. I don't know how the Americans found out about it, but, it was standard proceedure to challenge bombers which joint the stream by firing flares from the closest bomber. The flares were of different colours pre-determined (I quess by AF command). The joining bomber had to respond with a predetermined set of flares (again colour and number to be pre-determined). If the joining bomber didn't deploy the right flares, it was a fair target for the rest of the bombers in the formation.