There’s a lot in this. I think the best we can generally hope for as modellers is an acceptable range of a particular colour. I have recently been experimenting with GN loco blue on a HD body, and there’s an inexact sweet spot which just looks right based on taking some kind of average of published pics! My own experiments are a work in progress.
Ian Rathbone, who is the guru of model loco painting, says in his book that GNRI blue is basically impossible to define! And he’s painted a few. Two of his paint jobs below. One of these looks too light, but is what his client supplied. The other looks more on the money. Fred Graham’s were painted with genuine Dundalk paint but actually look a if anything little dark.