Something has stirred in the oul brain…I have a vague memory Mac Arnold may have advocated it. It’s easy to forget now but in preservation’s early days all kinds of weird and wonderful liveries could be seen in GB. In fact, even the RPSI’s 186 was initially turned out in SLNCR livery - plain black with red rods !
People had, I think, become so used to steam locos in appalling states of decay that anything which captured a bit of colour was welcome. NI enthusiasts had spent the years since 1948 watching locos gradually become a universal shade of dirty black. With no prospect of ever seeing the prewar NCC livery again, I think Mac may have advocated applying it to the Jeep, presumably on the basis they’d have carried it if delivered pre 39.