The colour on an individual vehicle could change over time. I remember an article explaining that when the lead white component of the paint was exposed to sulphurous industrial atmospheres, lead sulphates would form darkening the paint and conversely when the paint wa exposed to strong sunlight it would bleach back to a lighter colour. So a wide range of shades is possible, even before the effects of dirt are taken into account. There's a photograph at the front of one of the Midland Wagons volumes with two Midland brake vans in the background, nominally the same colour. One is pale grey, presumably newly painted while the other is almost black.