As a result of someone asking me "what is grey", I would clarify a point.
The grey most commonly seen in colour photos is the light grey used by CIE from the late 1960s until the brown became all-encompassing. NIR used an even lighter grey (as on Downpatrick's NCC Goods Brake Van) on PW vehicles only (as NIR never ran goods trains of their own) for a very short time in the 1970s.
But - and this one's important for modellers - the correct shade of grey to use in all other applications is much darker. The GSR, and later CIE, a well as the NCC and UTA, used a colour identical or as good as, to LMS wagon grey in England - this shade is readily available from model suppliers in that neck of the woods.
The BCDR used an even darker shade which can be seen on a rescued BCDR van body at Downpatrick. It would best be described as dark slate grey. I recall seeing a wagon in the 1960s like this, still marked "B C D R" and noticing how much darker it was. The GNR used a similar shade to CIE / GSR / CIE.
The SLNCR used a somewhat lighter shade of grey.
The lighter grey used by CIE on covered vans was not often replicated on opens, as the Bullied steel ones were very much to the fore from the late 1960s. Few wooden bodied opens obtained light grey, and fewer still brown, though there some examples.
Narrow gauge stuff was a mixed bag, as they so rarely saw a paint brush! The CDRJC had a lighter shade latterly at any rate, though in the 1920s some stock was painted black. The Cavan & Leitrim had just a small number of PW open wagons, which a century ahead of the modern Health & Safety Regime, were all yellow!
The GSWR used a dark grey which was almost black for much of its goods stock. When hauled (post 1915) by a plain slate-grey locomotive, this can't have looked very colourful!