IMHO  He had thin hair with almost a blondish color on the top and toward the back of the skull.  From the top down his hair git progreesivley darker.

Here is a link to a B/W where his hair color and the thinning of, was most notable

http://en.wikipedia.org/wiki/File:Bundesarchiv_Bild_146-1987-074-33,_Michael_Wittmann_und_Adolf_Hitler.jpg

Here is a colorized picture about a1/3 of the way down

http://www.militaryimages.net/forums/showthread.php?t=9362&page=14

Hope this helps

Looks like dark brown.

http://www.google.com/images?hl=en&safe=active&gbv=2&tbs=isch:1&&sa=X&ei=Wk9hTJ2fJ8G88gap8933CA&ved=0CAUQBSgA&q=Michael+Wittmann&spell=1

From the B+W photos I would guess black.