At our age, we are allowed the occasional brain-fart!
When I joined UBAS in early 1967, the fleet was silver with stick-on dayglo and had VHF radios. The new scheme and the UHF mod must have been applied over the Summer, which was when maintenance and painting was done, after Summer Camp (Binbrook that year) and before the Autumn Term, That would account for the colour scheme in the picture of WD353 in July 1968.