The simplest demonstration is to fly in no cockpit mode and look at the exhaust flame turn from blue to yellow during the climb; you'll see the same problem in the Spitfire & Hurricane too.
I think they just forgot the "AUTO" part of AUTO RICH.
As an aside, the large yellow flame is the result of after-burning, which is the continuation of combustion outside of the cylinder. This term is generally misused, especially by Americans, as a surrogate for reheat in the context of gas turbines, but it actually has, or at least had, a quite specific meaning in the context of piston engines before the turbojet was even thought of (on which note, today is the 74th anniversary of the first run of Sir Frank Whittle's engine, the 50th anniversary of Gagarin's space flight, the 30th anniversary of the first Shuttle flight, and, somewhat less significantly for aviation, my 27th birthday).
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