I think Sheldon is right to doubt that the white patch was a wartime
requirement. I believe that just pre-war regulations in Britain required a
red reflector to be shown in the middle of a white patch. The 1945 Road
Traffic Lighting (Cycles) Act actually gives dimensions for the size and
placing of the patch.
Confusion arises from the similarity to the white band around car mudguards. I have always assumed that to have been a legal requirement but it may have been advisory. Absence of chromium was caused by just that... absence of chromium! We did not have enough and there were more important uses for it than tarting up bicycles. There is an unused war finish Philips frame in the cycle shop of Amberley Chalkpits Museum in Sussex if anyone wants to have a look.
Actually, black is good. Rudge, among others, finished chainwheel, cranks and handlebars in black celluloid and it looks very smart. The advantage of nickel and chromium plating is that it does not chip like paint. I suppose I ought to add that in GB in the good old days too much chromium plating was considered rather vulgar. Stuart (Proud to be out of date) Tallack in bucolic Sussex