WWII German tanks were often neutral gray below the sponsons and buff above. Sometimes, parts were left in primer coat (red, not iron red, but an almost pure red), so you might have a red floor plate replacment next to a gray one, or red below the floor plates and such in the hull. I've seen black seat pads in most German WWII tanks... replacements? Originals may have been brown, but I don't know.
US Armor: White. Except open top (tank destroyers), when the fighting compartment (and sometimes the driver's area) were olive drab, or what ever the exterior color was. Seat pads were black, except for the OD canvas ones on tank destroyers, half tracks and such.
Instrument panels were olive drab with black faces on the gages. Gun breeches were usually gun metal (almost black steel color), while the recoil systems could be white (usual for a Sherman) or olive drab (usual for a later Stuart). Radios, a kind of half way color between forest green and olive green.
Engine compartments were also white.... Nice when the engine was out, eh? Probably couldn't see that white spot for more than three or four miles distant (shows the Allies owned the air!)
British Armor: Silver; to include instrument panels and such. Seats were a reddish brown. Radios, Gray??
Like the Major said, any open topped vehicles tended to be the exterior color inside. I'd imagine there may have been exceptions to this, but not many.
Hope this helps.
Ron.