Eric, interesting choice on the Mustang III.
I've had some correspondence, a few years back, with a Canadian who has been researching for about ten years now, to publish a book on British and Commonwealth use of the P-51. So I emailed him some questions for his insight.
From his response, I can say that escort missions over Norwegian waters occurred after D-Day (though of course Mustangs would still be weathered from escort duties over Europe).
"UK based Mustangs were normally repainted with British colours."
He also isn't aware of any other colours used other than the black and white for the invasion stripes.
"Temporary paint was used and could look pretty scruffy after a short period of time".
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Myself, I wonder if it did happen, it wasn't official and would have been a local decision to use something other than black.
The white nose front cowl section, I think this was a carry over from American practice, as the Mustang had been misidentified as a 109 during combat.
regards,
Jack