My understanding is that this was experimental, very short lived, and applied maybe to one or two of several types of wagons.
A but like green “H” vans, very few, and re-done in “normal” livery pretty quickly (by wagon standards!).
I do know that later, some cattle wagons, flats and opens (probably VERY few in the latter case) had small one-plank-height WHITE snails too - the cattle wagons shown is probably one of those.
The “light green” referred to was the standard “eau-de-nil” used with green liveries.
Since this EdN was long gone (from new paint jobs) before the early 1960s lighter grey wagon livery appeared, you won’t get light grey wagons with anything other than white numerals and snails (no matter what size).