Several points of clarification.
The letters GSR and GSWR were indeed often filed off; change of ownership precipitated this, but it seems to have occurred on a random basis.
NO cast plates ever had a black background under GSR or earlier CIE ownership, but see below, but in GSWR days they did prior to the introduction of lined black. Thus, the correct background for 90's plate at DCDR is black.
All Inchicore type plates should have a grey background for anything at all post 1917 or so, with the following as the solitary exceptions.
1. The few CIE locos which were painted black in the mid and late 50s - black.
2. The 800 class; both names and number plates blue, as on 800, until about 1952. After that, 800 remained blue while 801 and 802 were changed to red.
In all other cases, the same grey as body colour was used. My father recalls seeing some out shopped in late GSR and early CIE days with the surfaces of the lettering, numerals and plate rim simply sanded down to reveal bare metal, but most were painted a creamy light yellow, much the same as the colour CIE used for car side painted numerals. Sometimes rims of plates and numerals were painted a very light grey instead if cream, especially in GSR days.
On at least one occasion, I have seen a photo of a loco in CIE times with what looks a much lighter colour - possibly white - used on rims and numerals of cast plates.
It is important to note that once snails appeared, these were "eau-de-nil", not pale yellow, in other words a different colour to the cab side numeral.
Locos painted green in late CIE times all had painted numerals rather than plates, except the three 800s. When numbers were painted on green livery, as opposed to grey or black, they were in lined "Eau-de-nil", not pale yellow; i.e. the same colour as the snails on both locos and rolling stock, and carriage numerals.
As an aside, a small number of wagons - I think primarily horse boxes - got light green snails painted on their grey livery instead of the usual white... But that's one for wagon livery oddities....
The paintings on the front and rear of the book on GSR locos shows locos painted grey, with black number plates sporting red numerals, and black smoke boxes and chimneys. This is entirely incorrect.
Hope this is of assistance.