The Royal Scotsman train is meant to project a very traditional, nostalgic image though so it's just a case of painting the loco to match. It must be said however that class 66s do not suit traditional liveries such as this, in my opinion, and it is strange to see such a utilitarian freight loco given the "Royal Scotsman" treatment.
Personally I think that, on a purely visual level, the Royal Scotsman locos should carry a livery that differentiates them from the stock that they haul, rather than attempting to blend in: Royal claret would be well suited, in my opinion. I thought it made for an aesthetically pleasing pairing when BR green liveried 37403 was paired with the train in the mid nineties.