For the EW, I am pretty sure the finish was always originally black, for rim, nipples, spokes, hub, and brake drums (integral on the 350cc.) I have seen cases where Douglas painted over nickle plated spoke nipples, presumably purchased that way (nickled), up to 1936.
Douglas advertised that 24x2.5 tyres could be fitted to the 25x3 rims, but of more interest that 26x3 rims could be fitted at extra cost (presumably for colonial markets.) The latter being a size available today.
-Doug