They're long wheelbase, so unlike anything that ever ran here - but - For CIE livery, cattle wagons were plain grey all over: wheels up to roof, chassis, body and all. They didn't stay that way, like most wagons, and became grubby in no time. Roofs tended to darken a bit more due to exposure and exhaust fumes, especially in steam days.
Cattle trucks spent a lot of time in later years in outdoor storage, so sun and wind bleaching was a feature of their weatherbeaten appearance as much as dirt.
Some, but not many, were repainted brown after about 1971, but since cattle traffic was done away with in 1975, that wasn't long. If you want to go for that short brown period, I'd go for a ratio of one brown one to three grey, and at least one of the grey will still have a flying snail stencil. The brown livery, like the grey, was all-encompassing. Roof, body, chassis, couplings, buffers, the lot.
CIE "snails" or roundels were always on the left hand side looking at them. A small number had a stencilled message to the effect that they were "VAC FITTED".
With no British prototype looking anything like a CIE one, I am glad to hear that Provincial Leslie is planning one shortly. A long overdue and very necessary item for the Irish scene. I'll be looking for a load of them.....
I would add, re-reading your post, it is remiss of me to assume you're modelling CIE, as you didn't suggest you were! If you are modelling a British prototype, keep the black chassis but ditch the white roofs! Virtually nothing that ever ran on rails (though there WERE exceptions) ever had white roofs!
And, apart from that, a liberal coating of greyish-brown brake dust weathering. If steam era, sooty weathering on the roofs too.