what color was the interior and control panel, and also what is a Testors model master series substitute for german grey, or do they have that color. building it for the russian front

For a 'Panzer Grey' Vehicle - interior was the same..

For a 'Sand' vehicle - You've got it, - sand.

For an early war - initial stages of the Russian Invasion, PanzerGrey was used, later on the vehicle base coat was 'desert-yellow' with Camo oversprays.

If you can wait a few hours, I am going to be publishing a review of the new Osprey Publishing Book - Modelling the Sdkfz 251 Halftrack. This I believe could be the answer you are looking for..Jim

he's right there but if your doing a late war time frame
with an earlier version of the 251 you could combine a panzer gray interior and be perfectly right.

There's a pic in panzer colors of a 251 with a base of Panzer Gray covered with afrika mustard and then covered with whitewash the only areas left Pz Gray are the running gear and the area around the unit markings that you can see in the photo.

The Sdkfz 251 book by Osprey is outstanding. If you are going to build this vehicle it is a must have.
Recon

Here's the link to the review: 251 Book Review.. ...Jim