I mentioned that I had tried painting a GWR wagon with Farrow & Ball 'Rectory Red' paint.
To re-cap, this colour is described by the manufacturer as Vermilion mixed with Lead Oxide, to make it cheaper. It seems to me that could be a plausible formula for the red used during the 19th century on GWR wagons.
Because it is difficult to compare colours accurately between different photographs, I have taken a shot of two wagons together. .The one on the left is painted in 'Rectory Red' and the other is sprayed with red car primer.
comparison of wagon reds
I think that the brighter red, on the left, matches the 'light red' description, which some writers used to describe GWR wagons, rather well. Because of the red lead content, this paint would darken with age, which was, allegedly, the fate of early GWR wagons
Edit: I analysed the wagon colours in Photoshop, which shows that the oxide primer is more yellow (i.e.less blue content) than Rectory Red, as shown below: