Because of the short duration of such exercises photographs of these type of special markings are very rare or, in some cases, probably non-existent.
The Day Fighter Scheme (Dark Green/Ocean/Grey/Med Sea Grey), as the name implies, was developed specifically for the use on aircraft in the Day Fighter role. It was not used on aircraft used in other roles such as army cooperation.
Night was a specific type of paint, DTD308, not just black. (In fact it was not a true black as it contained Ultramarine pigment.) Because the markings were temporary I would guess that they used DTD441, a semi-permanent, washable form.
309 (Polish) Squadron was involved in Exercise Scorch. All that it says in the official history of the Polish Air Force is:
"During December 'A' Flight took part in the exercise "Scorch" operating from Ipswich, and 'B' Flight, in "Stag" operating from Inverness."
So there may have been a similar exercise called Stag taking place in Scotland at the same time.
There is a photograph of Lysander V9576 AR*N which has a black letter N under the starboard wing. There is also a photograph of the aircraft from the other side and the port wing could be black underneath.