It's War Department (or, as it says on the tender 'ROD' - Railway Operating Division, I believe) livery, as the loco carried not long after it was built (1917) when it was sent to work in France during WW1.
Having seen it yesterday, I can say that while interesting, it doesn't really match up to the gorgeous GWR green livery as worn by most of the other locomotives on display!
Yes, whilst its nice to see it for a change, I hope they paint it back to BR Green or GWR Brunswick before it leaves it ticket. There exists a photograph (albeit in Black and White) of the same locomotives near Chedworth on the MSWJR in the late 50s, and lined green suited it much better!
Incidentally, its it just me, or is it actually louder than Earl Bathurst? At the show in February they opened it on the demonstration line, and the noise it made was something terrific. Lovely machine.
You should have heard Lydham Manor on Saturday (or you might have done?) - it was volcanic!