Here's a very thorough account of the colours used for the interiors of German Panzers:
http://miniatures.de/html/int/camouf...-interior.html
There's an example at Bovington Tank Museum, the interior is actually Painted Ivory, Pale Beige - apparently RAL 1001 with Grey green floor (very similar to outside grey of vehicle) The stowage containers are all grey as per the outside and any equipment parts are semi gloss black. Appologies fot the quality of the pic but I was poking my camera against the view slit in the rear door - I just knew someone on this forum would want to know what it looked like inside!
Here's a link to some images of the outside of the same vehicle:
http://ampsnz.hobbyvista.com/realafv...51/Bov251c.htm
I would have thought that the Sdkfz 251 being open topped would have been painted the same colour inside as the exterior of the vehicle. It'd be nice if you could try to keep more of a factory look to the inside of the hull, just weathering the flooring and the outsides of the vehicle.