Based on various online sources I've read, the unlined mid-chrome green would've generally been used on mixed-traffic locomotives from 1906 to the BR era, although obviously -- as Richard points out -- there were exceptions. Lining and the garter crest was normally reserved for express passenger engines...live and learn.
I've ordered one in unlined green with "GREAT WESTERN" on the tender to represent 4326 as it would've appeared in the late 1920's-early 1930's. That particular loco, by the way, was not withdrawn until 1957, so it's possible that it would've carried any one of the liveries that Accucraft UK is offering at one time or another.
Last but not least, one very thorough source of information on how these locos were painted is at URL http://www.ianrathbonemodelpainting....es-1923-39.php