Hi Dan
I'll be the first to stick their neck out.
The story goes that Korvkpt. Schnee flew over a submerged U-boat and saw the dark shape under the water. When he was given command of the U 2511 he decided that it should be painted lighter than the then current schemes.
Just as likely it was an extension of his emblem (the snowman) so the sail was painted white (or off white) the upper hull Hellgrau 50 and the lower hull Schiffbodenfarbe III. This seems to be the generally accepted colour scheme. But I have come across the attached photo, it is possible from this photo that the whole of the upper deck and sail were the same colour.
Regards
Jon