Beware of Ken's models, he's confusing paints tint very often, he painted the Maxi 5 Turbo Gold (?!?) instead of sand "beige" for example so he's not always a perfect reference for colors (nothing to deal with his talent to build BTW) and I also think that most of his books references may not inevitably show real original cars but sometimes replicas...
I by chance have a lot of books and videos from the real cars collecting through the years as these fabulous cars are my fvorite rally cars ever.
Also pay attention to the pics taken during night stages, there is often a light effect due to the highlight flash of the cameras that show a black inside because of the contrast inside/outside the car.
All the pics I have (and normaly already shared) show a white inside for the body inside panels of the sport quattro and S1 (the group 4 car had a black inside).
I could also confuse myself but I've checked the following linked pics I have and even the roll cage is white:
Corsica
SanRemo
Most of 1/43rd scale brands did the same mistake and anyway for most of the brands maker when they can't identify a color inside they put it black !!
Maybe influenced by some of the pics one brand or modeler becoming a reference for others, at the final point almost everyone do the same mistake...
I feel a bit confused to be bringing it to the front of the scene but I thought it could be better for you to know and better to know that now than once finished the build