When I started as a staff cadet at 613 in '64, there were photo's of 'Mk 3s' with a yellow painted upper surface to the wing.
I was told this was to denote they had originally been Mk 2s ie tandem seat but with a parallel chord wing and had been modified by the substitution of the later Mk 3 wing with its tapered outer sections.
By the time I paid my first visit to Halton in late 1962, the colour scheme for all Air Cadet gliders had been standardised as overall silver dope with stick on dayglo panels, even the yellow trainer band round the rear fuselage had gone.