If there is a difinitive answer to this one then perhaps Roy Larkin can come up with it from his great collection of records and procedures of that time!
But this was war time and I would think that the objective was to get the lorry on the road as quickly as possible, as safely as possible, as reliable as possible and with the minimum done, but still keeping the crew as safe as possible. Only paint where it is essential.
So with those objectives in mind, I would expect to see any bright object painted over if it was going to be visible to an enemy. Anything not visible but made of a bronze, brass or copper and which is tucked away would not be painted as the sort of corrosion that you would have with ferrous materials which need paint protection would not apply so painting for that purpose would be unneccesary.
In later years, we used to say that "bull s**t baffles brains" and it could well be that some bright fittings were continually polished to look smart if that was a Regimental or Unit procedure.
Tim does have in his collection, a picture of a lorry that has been "highly bulled" for a competition - and if I remember, it is painted with glossy paint, too"
Tony