Warren
Issues regarding the colour of paint on models have been around for all of my 65 years and likely sometime before then and I think a lot of unresolvable discussion occurs, which perhaps I am guilty of contributing to. Personally I feel the basic green on your model is too dark, but I have never seen a green painted SECR locomotive other than 737 or others in preservation, so who am I to judge and as Adrian reasonably points out, if the customer defines the requirement and is happy then all is well. Notwithstanding colour conversations, a nice piece of work.
If you follow Scottish heritage matters you will be aware that we returned 419 to traffic last November, in what is called Perth blue, which is a lot lighter than the St Rollox blue which is what most people call Caledonian Railway blue. The research and specification was done by the Caledonian Railway Association, most specifically by Jim MacIntosh who has written the book on the subject, and we are assured that within all the available evidence that the light blue is correct, yet many have commented that it doesn't look right, nor do I, however I do quite like the look of it. This has a parallel with 737 in that 123 is in the museum in Glasgow and was painted in 1959 and its shade of blue is dissimilar to both 828 and 419, which is correct nobody actually knows.
Steph
I acknowledge your basic premise that 737 was painted at Ashford to the original spec, however consider the safety valve lever, a steel component subject to variable weather conditions and also daily immersion in steam which would condense on it promoting rapid rust staining. It would require considerable cleaning on a daily basis and even allowing for the much higher standards of locomotive care in the Edwardian period it feels a step too far. I wonder whether the Ashford paintshop in reading the spec could have been misled by the absence of a specific instruction regarding this item and therefore left it unpainted. I know, I'm just trying to justify my position. I have found in a book about the P class two B&W pictures of the then new 753/4, and it appears that the safety valve lever is painted rather than unpainted, what colour is open to question.
Regards
Martin