That "Sky Blue" signal is a classic example of a little knowledge being a dangerous thing, since it was followed by a corrective signal, in which "sky blue" was replaced by Sky Type "S." Sky blue was the designated underside colour for what we now call "drone" aircraft, like the Queen Bee. Code letters, for fighters, were never sky blue; they started as sea grey medium, then sky, that's all.
That link has several errors; B.S.381C never existed prior to WWII, it was B.S.381; this might seem a minor point, but 1 Sky Blue became 101 Sky Blue post-war, when all numbers were updated. 210 Sky never had a pre-war equivalent, and Eau-de-Nil is a red herring (if you'll excuse the mixed metaphor.) Sky Blue, also, was never known, officially, as duck egg blue, that was reserved for Sky, which, likewise, was never known, officially, as duck egg green. Sky Grey was not used on RAF fighters, either.
Fighter Command, most certainly, did not ignore Air Ministry orders, neither did the factories, and it's mischievous to suggest it.
The Air Ministry never used British Standards during the war; when B.S. offered their services, they were politely told, "Thanks, but no thanks," since the A.M. wanted to be able to amend colours at possibly a moment's notice.
Judging by the author's name, I'd venture to suggest that he does not have access to the many hundreds of files in our National Archives, which explains (but only partially excuses) the errors.
Edgar