I think that it is possible that the interior color scheme changed during production. Maybe the hull interior was painted differently than the turret? It is also possible that the Ka-Mi had some light interior color other than white, maybe based on IJN warship practices. For example, the US Navy used a pale green color for interiors on warships. I don't know what the IJN practice was during the war. Maybe there was an interior color other than white? Maybe a primer? I don't know.
The photo I posted earlier of the cutaway Ka-Mi from APG is odd because of the lighting. They placed electric lights inside the hull. This is why the fighting compartment and turret appear so bright but the rear compartment is darker. The front compartment is very dark as the cut-away was made into the pontoon, not the fighting compartment, and so was not lit at all. I still think this shows a very light color being used for the interior
Here's a few more photos, this time from Ka-Mis on Luzon. The hatch interior looks light, but this is probably from the lighting, not the paint color.
So far, I have not found the main APG evaluation report about the Ka-Mi at NARA, only some scattered photos in the combat collections and the APG collections. Maybe I will have more luck next time at NARA.