There might have been heated discussions about the a/f paint on German WWII submarines. I personally doubt that it was red plus a dark boot topping. Please have a look at Doug Martindale's article, in pdf-format:http://www.rokket.biz/models/modelsweb/rokket/u557/images/uboat_colours.pdf
Page 11ff ...
He refers to the 1940, 1941 and 1944 editions of the so-called ABB Nr. 31 which was the official paint regulation for the Kriegsmarine. I have checked all three of them, the 1940 edition only online, the latter two I have copies of, and there is no mentioning of red anywhere for the a/f paint of submarines. In other words, I agree with Doug as I have checked his references for myself.
However, I do not have any documents stating how things where handled before WWII. I have something for the IGN of WWI, but I haven't checked this for submarines, will do that later.
The editions of the ABB Nr. 31 do NOT state anything as "alternative" boot-topping or antifouling colour. But by the way, [off-topic], they clearly state the dark grey antifouling paint as RAL 7016, which is the same as the paint used for the boot-topping on larger surface ships (RAL 7016) AND the dark grey painted steel decks (RAL 7016). All the same shade, just with or without the antifouling ingredients ... [/off-topic]
And ... as stated by Doug as well, in another article just for U-47, the colours on this very boat changed. The upper hull and conning tower was painted "Schlickgrau 58" later in the boat's career. For more details have a look at Doug's article:http://www.modelshipwrights.com/modules.php?op=modload&name=Sections&file=index&req=viewarticle&artid=870
Hope this helps.
Happy painting ~ Olaf!