Hi Frank,
I don't know if you checked out Dougie's U-boat Paint site, but he gives the equivalents of the u-boat paints in different paint brands, including Humbrol.
http://uboatcolours.mysite.wanadoo-members.co.uk/
For the upper hull and CT:
Light grey: Humbrol 127
Lighter grey, almost white: 147 or 166.
Medium grey: 145.
Darker grey: 67.
For the lower hull:123
The decks are subjective. They were wood covered in a black preservative. This would lighten to a grey, brown with the salt water and sun. I, personally, paint it black . Then, with oil paints, paint it different ranges of grey and brown. Think of the salt water bleaching parts of it and all the scuffing from walking and moving provisions around. I have also seen people paint it the same color as the lower hull too.
You can always check out my VIIC to see what I mean:
http://homepage.mac.com/jess13/PhotoAlbum44.html
Hope this helps.
Take Care,
Jess