I had thought of that and reset the Scenario to Summer fine weather. This photo shows the result. The whole cab now has a blue cast so the green walls looks the same as the grey metal parts and this also includes the loco dome outside the window and even the distant Goods Shed building. This may be related to Unreal engine handling of sRGB textures so I also tried turning the sRGB off on the Base Colour texture (making it Linear Colour) and the next photo shows the result.(In the rain) Although quite bright the colours are closer to the original with clear separation between the green walls and the metal parts and even differences between the cast valves and the lighter coloured steel parts. The main problem with this version is that the green walls have large areas of "oil stain" colours which I guess are due to noise masks which I applied in Armor Paint (to get the colour variation) being too low resolution for the 8 bit 4K texture to handle . I saved a 16 bit .exr version of the texture but this was 95Mb compared to 6.5Mb for the 8 bit. I'm not sure if I need to have a Post Processing Volume (or Lightmass) in the Cab but the DTG models I'm copying don't seem to have them. I 'll leave it as it is at the moment and concentrate on trying to get the controls working.
Thanks to you and star#5823 for your encouragement. I'm not sure I'm really third party partner material and for the moment I'll just carry on trying to sort things out for myself. Thanks again for your support.