When BR took over the railways ???1948 they had mixed stock with mixed colours. The larger Peppercorn and Gresley Pacifics, the Stanier Pacifics and some Navy class, along with many of the 'Kings' class, were painted in dark blue (later changed to lighter blue for long haulers), some of which ended up on "Royal Scot" duties.

Take a look under "Liveries" on BR takeover