Re: Halftrack interior, what color?
It appears that the forward section with the driver and co-driver seats was initially painted light cream as in most other German AFVs. Here are some interior views of the Ausf. C: http://www.pietvanhees.nl/251/03-standardlayout.htm
I've yet to find a clear wartime picture to prove it, but it would probably be very likely that after the switch to dark yellow the entire vehicle was painted with it, inside and outside.
The Sdkfz 251 was produced throughout the war, but with changing versions. The most common were the Ausf. C (1940-43) and Ausf. D (1943-45), with the latter one being somewhat simplified to ease construction. If you have an Ausf. C, the interior would most likely be left in the original grey. Many units didn't even bother to completely repaint them on the outside, but applied a camo pattern over the grey base paint instead.
I hope that's some help to you