Good morning Dora-9
That is certainly a unique scheme for a serving UK Mil Chipmunk. What's particularly interesting is that the usual 'Anti-Dazzle Black', forward of the front cockpit windshield, which I believe was introduced early on as a Flight Safety measure, has been overpainted with the mysterious colour
I did also think at first glance that the lower section of the engine cowl, below the 'cheat line', might be a different colour to that of the fuselage. Likewise the rudder ... but that might just be the position of the sun/highlighting ?
I note she is minus Spinning Strakes and is a Glider Tower ...
Hopefully someone will be able to help
Best ...
Coff.
PS. As a pure guess ... could the colour be white/duck-egg blue (yes ... its a B&W pic Coff !) ...