Try this:
Primary sources
Allgemeinen Baubestimmungen (Building Regulations Order) Nr. 31 (March 1940)
Allgemeinen Baubestimmungen (Building Regulations Order) Nr. 31 (November 1941)
Allgemeinen Baubestimmungen (Building Regulations Order) Nr. 31 (July 1944)
Secondary sources
Gannon, Michael. "Black May." Aurum Press, 1998.
Högel, Georg. "U-Boat Emblems Of World War II 1939-1945." Schiffer Military History, 1999.
Jones, Daniel H.. "Kriegsmarine Camouflage 1939-45"
Jung, Dieter, Abendroth, Arno and Kelling, Norbert. "Anstriche und Tarnanstriche der deutschen Kriegsmarine" (Painting and Camouflage of the German Navy). Bernard & Graefe Verlag, 1997.
Paterson, Lawrence. "Second U-Boat Flotilla." Leo Cooper, 2002.
Showell, Jak P. Mallmann. "7th U-Boat Flotilla." Ian Allan Publishing, 2003.
Showell, Jak P. Mallmann. "U-Boats In Camera." Sutton Publishing, 1999.
Stern, Robert C.. "Type VII U-Boats." Brockhampton Press, 1998.
Stern, Robert C.. "U-Boats In Action." Squadron/Signal Publications, 1977.
Werner, Herbert A.. "Iron Coffins." Cassell Military Paperbacks, 1999.
Maybe this will help: http://uboatcolours2.mysite.wanadoo-members.co.uk/