Hi Ed,

Page 112 of Anstriche und Tarnanstriche der deutschen Kreigsmarine shows U13 in 1935/1936 with a white tower and light grey number (sometimes dark grey number). At this time they had a squirrel grey upper hull rather than the light grey. The lower hull would have been the normal dark grey RAL7016.
Squirrel grey was RAL7000 (FS35237), and was also called fehgrau (fur grey) or Dunkelgrau 51. It was used as standard on the upper hulls of Kriegsmarine warships and was a medium blue grey colour. Humbrol 145 or Colorcoats KM02 are good matches.
By 1937 or so, the towers and upper hulls of Type IIs were painted in the standard light grey, with their number painted in white.

Cheers,

Dougie