The GSR's wagon grey was the same pretty standard stuff used by the GNR, DSER and NCC also. BCDR grey was slightly darker. The C&L ballast wagons (of which I think there were about 3) inherited the sandy yellow colour from the C & L itself. Lettering was black on these instead of white. To my knowledge they were the only exceptions to the normal grey anywhere on the GSR. There's a pic of one in Paddy Flanagan's book.
The GSR, with it's apparent love of all things grey, painted all PW, loco and other departmental stuff the same grey as goods stock. I think, but I'm not certain, that the weed spray tanker was black, with support vehicles grey.
CIE continued this until the advent of yellow for things other than track machines, into the 1980s.
A point here - if anyone is modelling a weedspray train in anything other than recent 2 decades, it may be grey, not yellow!