Could it simply be that the frame has been resprayed by someone in the equivalent of BSA polychromatic beige"?
I have what I believe to be a NOS "cow pat" nacelle in beige. The OEM painted nacelle is not fitted, as my bike is currently black, and it seemed a shame to respray it, even though I had to buy another pretty poor condition one for $$$$$ just so I could put it "on the shelf" as a spare or sell to someone in great need of it.....bikes I've seen restored in beige tend to be more gold coloured than the nacelle I have, some bikes are VERY gold coloured
The nacelle colour matches the underneath of the bottom fork yoke on my bike, which I believe also be original paint, I did not paint that part of the yoke, just in case someone ever wanted to put it back to its original beige colour in the future, they would be able to colour match to the yoke.
One thing though, how original would the beige paint colour be on any part, after 60 years, even if it has been kept out of the sun etc?
In the pic The nacelle without the rubber trim has the OEM paint, the other is someone's effort to match the original paint.