Sorry guys, no red on wartime Type IX subs. Some Type VIIs early on, but by 1940 the Germans had started painting lower hulls dark gray, upper hulls a lighter gray.
I typically use Tamiya German Gray and US Neutral Gray for the colors. I add some light gray to the German Gray, and a touch of IJN Gray to the US Neutral Gray to give it just a hint of blue tint.
Here's how it looks on my Amati 1/72 Type VIIC U-boat. I know, Amati doesn't make a Type VIIC. I converted it, here's the article:
http://www.modelwarships.com/reviews/ships/dkm/u-boat/TypeVIIC_build/amati.htm
Regards,
Jeff Herne
Modelwarships.com