The dark green currently on 90 is a close match to T&R Williamson's RAL 6007 (roughly RGB 44, 50, 36). Not sure about the red or the light green on the lining. The dark green (and the rest of the livery) is based on this contemporary model of a GSWR loco which is at the Science Museum Institute of Mechanical Engineers in London.
As above the plates on 90 were designed and made by IÉ out of brass GM keys. They were donated to DCDR during the 2009 Inchicore open day and were fitted to the loco about 2013 I think. The originals were probably stolen/lost while it was at Mallow in the 1980s, although IMO both of them are very much part of the loco's history. Interestingly, IÉ made at least two other plates to the new design – another number 90 and a number 60 (easy to make as it's just 90 with the 9 upside down), which are both in the Wheeltapper Bar at Fitzpatrick's Hotel in New York.