She was plain grey from build date until about 1951, when she was repainted green for her last 4 years or so.
The grey livery depicted on both front and rear covers of the book is incorrect in two significant ways. While the shade shown is more or less correct, the smokebox and chimney are shown black; no GSR engine was ever like that, and no CIE one either; all was grey. There's always an exception to every rule; a single Cork-based J15 was repainted in its normal grey as late as about 1960/1, but with a black smokebox. This was both unique and very short lived - the loco in question was withdrawn 18-24 months later.
The other incorrect detail, despite an assertion as to its validity in the text also, was the numberplates, shown as black with red letters. Numberplates were also always grey, usually with numerals picked out in pale yellow, though sometimes just painted over and occasionally bare whitemetal as on 186 currently.