.
Still no primary source (a yahoo group isn't one, and the contemporary ad makes no mention of a coronation connection - just that Petre was picking colors).
That said, I sense that the "popular belief" (see "folklore") on this subject is strong enough that resistance is futile! Here's to Coronet Cream, inexplicably the only one of a range of colors offered by Austin in 1953 to commemorate the coronation of HRH Queen Elizabeth II. [And as they say in Wikipedia: "Citation needed"]