They were that colour when brand new, but a bit like a silver A class or silver tin van, or a steam engine in a light or bright colour, they got so filthy, so quickly and so inevitably that the actual livery they were painted in become lost under a layer of gunk.
Pretty soon they were repainted in the standard brown colour; this concept also applied to other vehicles like the Castlemungret yokes.
In the very few cases of Irish wagons having a livery other than standard CIE, almost inevitably while they LOOK "private owner", they are fully CIE / IE owned. The "bubbles" were branded "Irish Cement" towards the end, but were always owned by CIE - same with the Taras.
Actual fully private owned wagons - we're going back decades, and then not many. I'm sure others will add to the list, but on the BCDR the East Downshire Co. had a few coal wagons, and oil companies had a small number of tankers in the 1950s and 60s.