thanks for chiming in on this. I somewhat suspected as such as I know a few 1:1 restorers as well, and they take thier research pretty seriously. I became suspicious when the only places I found white interiors was in museums and on models. Even the Kruse ambulance has a white interior (great for taking photos though!)
I noticed the more I searched, all the "serious" restorers had the interior painted the same as the exterior, and that included several shots of unrestored vehicles that still had origional leather seats!
I also figured that even in black and white period photos, a white interior would show up pretty well, even with shadows. Havent seen that yet.
Now that is not to say some stateside USN ambulance might have had a white interor...but I would still lean toward the Navy Grey of the exterior.
Besides...what a pain it would be having to keep that white clean! Especally on the front line..
Thanks again, and I hope I do this model WC-54 justice. Feel free to chime in if you see any details off or something that could be added. My references are slim (mostly what I found via several google searches).