Hi,
There were three choices of upper colours for U-boats -
Hellgrau 50 (light grey, MM1728)
Dunkelgrau 51 (medium grey despite the name, MM1721)
Schlickgrau 58 (medium to dark grey, don't know MM code for this one, sorry)
The lower colour was a dark grey antifouling paint -
Schiffsbodenfarbe III Grau (dark grey, MM2101)
Wooden deck was coated with a black wood preservative, not painted.
For more info see -
http://uboatcolours.mysite.wanadoo-members.co.uk/
http://uboatcolours2.mysite.wanadoo-members.co.uk/
Enjoy your build,
Dougie