Hi Keith,
The nose cone in front of the actual warhead was not painted and it also was kept grease-free. It contained the "Storch" (=stork=2 [T V] resp. 4 [T XI] 'ears'= sound listening devices). I also have not heard of painted torps (as per your description)in the KM, except that the warheads were painted grey. Main body until '39 unpainted bronze, thereafter unpainted steel - all well greased.
When I saw U505 last in the middle sixties, I do not recall the torps painted. Am I wrong? Rgds, RB