I have been asked, and sometimes see questions, about weathering colouring of white coach roofs. I have ploughed through books and looked at photographs and decided that in the days when such roofs were in mainline service they discoloured fairly evenly and quite quickly. I have seen many references to such roofs in preserved service and it seems to me that they do not weather in the same way because their use differs.
This is a personal view!
I have attempted to replicate this process using Railmatch Roof Dirt, the tone of which has been randomly altered by adding Weathered Black at times, thinned a lot with white spirit, and applied in very thin layers from about 30cm away. The results: