Thanks for the replies, guys.
Good thinking Modeldad, you may well be right. However I have also seen it in reports of 359th FG pilots fighting against JG 27 (24 March1945 - see:
http://forum.12oclockhigh.net/showthread.php?t=4661). I don't know of any 109s with such mottling, but it's a plausible explanation.