True, but in the case of the railways they tended to match it. That said, differing qualities of paint were another matter; you could paint two things the exact same colour at the same time, but if the paints used were of different qualities or compositions, one would fade differently to others.
Especially in the 1950s, with wooden sided coaches, this gave rise to the urban myth that no two greens were the same. On the day they were painted they were all the same - one of the two shades used pre- and post-1955. But as they wore and weathered and faded, they did it differently, thus ending up all sorts of shades. On wooden bodied coaches, the older variety of green darkened, while the post-1955 colour became just very washed-out looking. If you look at Downpatrick's TPO and G611, you'll see this clearly. They were painted the correct shade when first restored, but they're not now! For once, its not due to preservationists getting the colour wrong (although G61X class were never green!) - it's just due to weathering.