BR bauxite is indeed close enough for the brown. After new painting it adopted a much flatter hue very quickly, as old photos (and recollection!) will testify. In more recent years, post 2000 at a guess, it shows a reddish hue when newly painted, but again this dulls down, as a quick look at any Tara timber wagon shows.
The grey used by (initially) the GSR, and on into CIE days until probably the mid fifties or so was very much like LMS grey, and may have been exactly, as their carriage maroon post-1935 was. The NCC also used a shade like this, as did the GNR. It was fairly standard.
The very much lighter shade started appearing around the very late 50s of early 60s, and would initially have been associated with the then current production run of H vans, though earlier H vans were the darker shade. This lighter grey was also applied at this stage to pallet vans, repainted wooden open wagons and guards brake vans, the latter having yellow and black flashes on the ducket (NOT white and black, as on the one in Cultra!).
How to describe the shade... Grey is always a hard colour to describe! I would not be the expert on reference numbers etc, but I'll see if I can find a suitable picture to illustrate it.