GWR green was not Brunswick Green. There will be a paragraph on this in the forthcoming HMRS book on early BR SR liveries. The author states:
"A group of slightly bluish greens was assigned the name “Brunswick” (qualified as “light”, “mid” and “deep”), based on the name of the part of Germany where the pigment associated with the colour was first produced. The 1948 edition of BS 381C: “Colours for ready mixed paints”⁶ allocated numbers 225, 226 and 227, respectively. These greens do not represent GWR or British Railways Standard Locomotive greens. Both GWR and BR greens contain more yellow in the pigment mix than represented by the Brunswick greens and the author understands that the GWR colour upon which BR based their choice of green was properly known as middle-chrome green.
This of course was the green adopted by BR."
And although in earlier days the railway companies mixed their own colours, in more recent years they bought in paint from a number of manufacturers which produced it to the railway company specification. By Nationalisation this was the rule, and of course new types of paint were coming into use.
And the comments about lighting are apposite. You need to paint your models under the same lighting as you use to light the layout, and preferably the same kind of light distribution - direct, diffuse etc.