Hello,
I managed to track down a thumbnail of this slide but the colour was not clear. To be fair the loco is so dirty that even the slide probably doesn't show the colour very well. The GWR lettering looked rather indistinct suggesting that is was showing through with the overlying paint having worn off (possibly assisted by over-enthusiastic cleaners ).
If I wanted to model it, I would probably start with a black loco as the base, remove the BR crest, put GWR transfers on and then work to distress the lettering. Lastly I would weather the whole thing heavily.