Donald and Douglas were built by John F. McIntosh at the St. Rollox Railway Works in Springburn, Glasgow, Scotland and worked on the Caledonian Railway in Scotland, a large network that extended along the country's east coast as well as the central and south-western regions. Despite being limited to Caledonian metals, they appeared to have a familiarity with other railways, as they were familiar with the locomotives and livery of the Highland Railway.
Giving long and stalwart service, Donald and Douglas eventually became 57646 and 57647 on the Scottish Region of British Railways. In 1959 Sir Topham Hatt needed a goods engine and by his request, BR transferred 57646 (Donald) to Sodor.
According to the Railway Series, Donald and Douglas were painted black from construction and arrived on Sodor in the BR's black goods livery with red-and-white lining on their tenders. They remained as such (with the addition of numbers on their tenders) until it was decided that both engines could stay, at which point they requested to be repainted in the NWR's blue livery with red-and-yellow lining. They apparently like the colour, as it reminds them of the Caledonian Railway, where many of their shedmates carried a similar shade of blue.
In the television series, the twins arrived in BR black and have remained in this livery ever since