References. Office Naval Intel pubs:
ONI 220-G (4/13/44) German Submarines
ONI 220-GE (4/13/44) German Submarine Equipment.
Above courtesy of HISTORY ON CD-ROM, and their CD-ROM N555.
Although a few U-boats were extensively camouflaged early in the war, very few cases of deceptive patterns have been recently been reported. Generally U-boats are painted to blend in with the light and water conditions in which they are operating.
Arctic - White or light grey.
Atlantic - Medium-dark grey, sometimes light green,
brown-green or green-blue.
Mediterranean - Light grey, sometimes in an alternate striping
pattern with green.
Please note. This is only one (or two) reference(s), and by the US Navy at that. You might also want to search and check German references and sources too.