Nothing was ever either white or off-white - everything was the same eau-de-nil. Most of what appears whitish in photos is the deterooration of the photo (or low quality of iriginal film used to take the picture*), and the rest would be due to fading, like the door numbers on the MGWR 6-wheel first that was on the Loughrea branch in the late 50s.
(* Sadly, while Cyril Fry was a great modeller and also took a number of highly interesting pictures of some very interesting subject matter, he scrimped and saved with colour film! Much of his colour stuff - in fact, most of it - is so poor now that it's beyond redemption - there is very little that reliable livery details could be gleaned from, if those doing the gleaning didn't already know what was correct....!)