Actually, my limited resources (RP Hunnicutt Armored Car book and Squadron US Armored Cars in Action) show the interior is white. For the M8, the interior is white below the turret ring with an OD floor. Even the 37mm ammo rack is white.
There are some really nice photos of a restored M20 that clearly show the white interior even up to the edges of the superstructure, but since this is a restored vehicle, the colors could be suspect. The floor is OD like the M8, but the bench seats look OD or some other dark color. Inside of the driver's hatches were OD as well.
I have the Tamiya M8 and the Italeri M20. Big difference in quality. Not that the Italeri kit is bad, it's just that the Tamiya kit is better.