Hi Kit .....hmmm!

My suggestion on paints, too use on U-boats kits is Enemel "matt" finish paints.
The Robbe U-47 is a big kit, so a proper colour spraycan should do the trick.?
Most German U-boats did have a two tone colour scheme...Below the waterline was a dark "matt" grey finish & above the waterline including the conning tower was painted in a light "matt or flat" grey finish. The deck may be in a mud-like grey finish, as most parts of the deck was built from wood. As for the weather/rust looks, use a flat semi-hard brush dip in with a little of rust/brown colour paint and make straight little lines down the conning tower area and around the U-boat's sea vents...but only use the brush a little at a time..
I hope these tips help.? Have Fun!

Rds Joe