Hi Dan,
the colour used was not that dissimilar to the Crimson Lake colour used on LMS (London Midland and Scottish) railway locomotives. There has always been a healthy rivalry between the Great Western Railway (GWR) and the LMS, so to see a GWR locomotive (a Hall class) which should be resplendant in its GRW Green and Black scheme, in something that looked vaguely LMS, was a bit of shock! So I was taking a tiny little pop at the LMS......
That said, it could have been a historical fact. The LMS were so impressed with the performance of the Castle class locos, they wanted to buy 50 of them, but the deal never came off. The LMS designed their own near-equivilant, the Class 5, which was a very impressive and capable loco, and they made over 800 of them; more than 5 times the number of Castles built!
It is true though that GWR engineering standards set the level for British railways; other factories made new locos with tolerances at the same level where a GWR loco parts would be scrapped. This meant that wear and tear was much less of a problem and GWR locos generally spent far more time working and much less time being repaired.
So many other factors had an impact on loco design and performance; GWR had access to Welsh coal which was much better for loco boilers than that used by other railways, for example. There are loads of good books on the subject, but the one I mentioned above (British Steam Since 1900 by W A Tupin) is a good primer on the subject; it turns up on eBay quite frequently.
Tim