Tom,
Go with the blue tunic and black trousers. Andrew Mollo's "German Uniforms of WW2" states that the city fire brigades weren't unified as part of the police until 1938, when they received this uniform. Don't know where the grey comes from. To back this up, he shows female personnel under training in 1944, they are wearing a grey overall, but the instructor is clearly wearing the blue/black uniform. Part of the difficulty in fire brigade clothing, is that so many members of other organisations, e.g.HJ were co-opted into service in the fire brigades. The regular members wore the blue/black uniform.