I strongly suspect Doncaster cab interiors were the same colour as the outsides, but I have no absolute proof.
What I can say is that 'light stone' is not a Doncaster tradition, I do have evidence to prove that.
Jonathan your 'red' theory is interesting, I wonder where it came from?
The details of livery are often fascinating, I remember a series of articles covering LNER locomotive livery in the MRC, I think, in the late 1970s, a good deal of the information in them was dubious. But once it is published in a respected journal and models made using that information, it becomes 'fact'. NEVER use a model as livery evidence, unless it was built by a contemporary maker with a job at the works in question.
I was the LNER writer in the 'Big Four in Colour' David Jenkinson volume, we certainly didn't get everything right. And I have collected more evidence over the years.
John