The underside of the commander's hatch and interior walls would be an ivory color. The inside of the loader's hatch would be dark yellow (since it was visible whereas the commander's rotating hatch would not be).
Basic rule of thumb: if a hatch opened and could be seen by a potential enemy, it was camouflage colored. There were rare occasions where exposed hatches were painted a bright color. These were foolish exceptions. But modellers seem fixated on these. How often have you seen a Panzer IV or Sherman tank where the underside of an open hatch is painted white by a modeller? Too often -- and too inaccurate, IMHO.