What an excellent illustration of your point. And they say there is no evidence?
I had a look through Great Western Branchlines (CW Judge), which has some interesting photos from the Edwardian period (although often undated and not the best quality). FWIW:
Canopies: Three-colour scheme evident on canopies at Devizes (dated 1930s, but it looks earlier to me) p23, Chippenham (no date) p23, Brixham (1910) p41, Helston (no date) p56
Canopy glazing bars: The Brixham photo has a (not very good) underside view of the canopy glazing bars which to me are clearly not chocolate (see also canopy at Darthmouth on page 43 although that could be later in time).
Poster boards: Page 45 has a photo with several poster boards in the Edwardian period. These have "Great Western Railway" in either gold or dark stone across variously 2 or 3 poster bays, with the lettering on a chocolate background. *But* the framing is clearly a stone colour, probably dark stone.
Striped valances: Barnstaple (p36) and Brixton Road (p47)
Regarding poster boards: Would these have been made (and thus painted) on the spot or pre-fabricated elsewhere to a standard design?