I would agree, Minister.
While an owner of an old vehicle can paint it any colour they like; a wrongly painted artefact can always be changed when suitable info becomes available, and quality info is not always available, and nor is everyone that interested in a colour; it pains me to see something wrongly painted because it becomes de facto "wisdom" that it actually did look as it does now, when in use.
There are examples of things in all three of the 5ft 3 preservation locations which have been wrongly painted. In some cases (RPSI coaches) this is deliberate. The Dublin blue livery was specifical designed to differentiate the Society's Cravens from IE ones, or other IE stock. The Whitehead ones were meant to have a livery which was not unlike UTA, but not actual UTA. In both cases that works well. The former dining car No. 87 at Whitehead spent its entire life in UTA green until 1967, having been built several years post-NCC, but it looks well turned out in NCC maroon; the coach is of NCC design though a UTA coach.
But restored goods stock is another thing entirely, with but a single example that I can think of (the Donegal open wagon in Cultra) being correct. Just about everything in Cultra which has been painted there, rather than outside, is not correct.
Thus, in painting a very old carriage, the greatest effort should be made (in my opinion) to get it right. But in the case if this vehicle, and (for example) the Castlederg carriage in Cultra, it has to be recognised that exact information may not exist.
While this discussion relates to a full size vehicle for restoration, it should equally be of interest to modellers.