Just to throw another pebble into the pond, I have been puzzling for some time over the correct colour to paint various BR-period NPCS.
In theory, these vehcles should, I believe, be 'crimson' ['carmine'?] in the early BR period, and 'maroon' later. I assume the early colour was the same as that used on non-corridor coaches [but was that the same colour as used on the lower panels of bogie coaches painted in the 'blood and custard' livery?].
An added complication is that colour photos often seem to show these vehicles carrying what looks for all the world like red oxide (i.e. the BR fitted freight livery). I think I have the answer to this. A colour photo of an ex-GWR autotrailer in the early-mid 'fifties seemed to show this same colour, from which I deduce that the 'crimson' livery (whatever it looked like when first painted) tended eventually to weather to a reddish-brown, maybe not quite the same as red oxide, but certainly a rather 'faded' red.
I suspect this may be what gave rise to the original query about the the painting of meat vans. If painted 'crimson' initially and then allowed to weather and fade, the result may well have borne a superficial resemblance to the red oxide fitted freight livery.
I have not yet decided precisely how to paint my BR-period NPCS, but I will definitely try to introduce a hint of 'red oxide' into the colour, even if would have been 'crimson' to start with.