I have a 1928 Flyer and I have researched the petrol tank colours. The answer depends on whether yours is a 1928 season (available from late 1927 until late 1928) or a 1929 season bike (from late 1928).
The 1928 season bikes appear to have had purple side panels and a purple diamond on the top of the tank. This was like lammy 1’s but had a different transfer of an actual Flying Squirrel on the tank sides. See page 48 of the (first) Scott Selection for examples.
The 1929 season bikes had white side panels with the Scott Emblem transfer and a white triangle on the top. Page 60 of the Scott Selection shows examples of these.
Now, here is the rub. It is not nearly so clear when the change in colours was actually applied. My bike, which was first registered on 26 April 1928, now has the purple panels courtesy of a rebuild and re-paint in the 1980’s. However I have photographs of the bike as found in a barn before restoration with white panels. It does not appear that the tank had been changed as the auxillery side-wall oiler used on the 1928 models remains in situ underneath. The tank could have been repainted with white panels sometime in between but I doubt that this would have been the case (waht would be the point?). By the by there were also 1940’s style pin-up girl transfers on the tank of the sort applied to the side of war time bombers!
The transition between models (as appears the case with a number if not all Scott models) must have been gradual. On page 96 of the Second Scott Selection is a picture of a Flying Squirrel registered in September 1928 with white panels and Webb forks but the earlier aluminium exhaust manifold of the 1928 model.
I stand to be corrected if anyone has anything to add (!!).