All MGWR coach chassis were black; in fact, as far as I know, with the exception of a few CIE new-builds in 1955/6, all Irish passenger-carrying vehicles, “full-stop”, had fully black chassis.
When the MGWR’s dark lake (very dark maroon) came into being in 1918, I have no clear information about roofs, but they were probably painted in the white lead, which darkened almost immediately and then got blackened with smoke. Carriage ends were maroon, just as carriage ends had also been brown when that livery was the norm.
So, yes, a dull grey would suit carriage roofs well.
That actually raises an issue; when general carriage roofs in the 1890-1920 era are described as “white”, e.g. with the GNR, it was actually actually white lead paint, for waterproofing. That was in fact a very pale grey rather than pure white. We see white roofs on model WAGONS all the time (thanks to Hornby!) but this is inaccurate for Ireland anyway.
The state coach did indeed follow the norm. It skipped GSR livery and remained in MGWR maroon until it got its first and only cost of green in the late 50s, just a couple of years before it was inexplicably scrapped! There’s a photo of it in MGWR livery on P96 of Ernie Shepherd’s book on the Midland.
The late Des Coakham described it as being by that stage “a dullish red”. The roof is clearly a darkened grey.