Hi Chris,
Do you have the 'Middleton Press' book', 'Branchline to Wantage' ? It contains about a dozen views of No.6, which is pretty well all there are.
In the majority of in-service views there is much less tonal contrast between the body and the skirts than you would get using green and black on a 4mm model, but this could be due to the film response .
However, Plates 8 and 89, taken in 1920 and 1930 respectively, show the loco in a run down state, where there is some difference between the body and skirts, and could be evidence that the colours were different; and black skirts would be a reasonable suggestion. For the 4mm model, there might be a case for some judicious weathering, or what the military modellers call a 'filter' to reduce the contrast.
Other issues:
By 1920, No.6 was in need of attention... the paintwork was very faded and damaged. The lining is very faint, but it is possible to make out that the corners have inner facing points, as on earlier views, eg Plate39. Also, there is no plate above the coupling shackle on the buffer beam.
By 1923, Plate 7, the loco has been repainted, and relined, but with inside curved corners. There are some changes to the buffer beam. The horizontal split, visible in most earlier photos, has gone, either filled, or a new beam? There is a new 'bracket' on the right side; and an oval plate in the centre. I suggest this is not a makers' plate. I think the refurbishment was carried out at Swindon, and this plate relates to that. No.5, 'Jane' had a similar one. It depends on your client's choice of time period, but the post 1923 changes only lasted for two years before passenger service ceased, and the loco was dumped.