I don't know whether the primrose yellow of the Wanderer is a Rootes colour. Our 1967 Wanderer was yellow and I have seen many other Wanderers in the same shade of yellow, but not on any other conversions. When I checked on the DVLA website, ours was listed as white. I wonder whether it was a colour applied by Bluebird, later CI who did the conversion and if so, whether there is any way of finding the code. [?]
Of course, if you want to change the colour to another Commer colour this is not a problem.