How common was plain green livery on BR steam locos, particularly on the western region? It seems most current RTR models of BR era steam locos are either plain black, lined black or lined green? Is this a gap in the market, or just a reflection of how things were?
Pretty uncommon in my experience, but towards the end of steam there were some economies made when locomotives were given works visits which included repainting. From probably around 1964-5 many locos (both green and black) which had previously been lined, were turned out in plain green or black. I can certainly remember unlined (but still green) 'Jubilee' 4-6-0s, but even then it was the exception rather than the rule.
Lined green was used more towards the end of WR steam as a cost-saving measure, though I think some locos such as 57xxs and Large Prairies may have received it in the late 50s/early 60s (maybe because such engines weren't considered important enough to be lined out?). Locos which had unlined green later in their careers included BR Std. 4 4-6-0s and Std.
2 2-6-0s and Ivatt 2-6-0s - many of the engines from these three classes so treated were based on the Cambrian. Away from ex-GW territory, some of the LMR Britannias which lasted until the final years of steam were also turned out unlined as repaints were done "on the cheap".