According to the 1940 regulations, wheels and handles used for quick-release vents had to be painted red, in the 1944 regulations they also allowed green.
Source:
http://www.u-boot-archiv.de/dieboote/farben.html
The exact location of the handle is hard to see on the pic. As it is located on the top, it could have been used for the fersh air conduits to the diesel engine or the internal fresh/used air pipes. Those start just start aft of the observation scope - close to the air vents for dive bunker 2 and 4.