I think the problem is more subtle, and will be familiar to anyone who makes and paints plastic models.
The actual colour chips are the correct colour – they have to be. But they don't look right as chips or on a small test piece.
That's because we don't see chips of paint, we see full size locos in full, natural daylight, but we see test pieces and models under widely differing conditions and on a totally different scale where shadows, specular effects and reflections create a very different perception.
Add to that the 'mental image' of the correct colour which is probably heavily biased by generations of cigarette cards, book illustrations and paintings, all of which will actually contain a whole range of colours to give the impression of a loco painted in a single colour with depth and further affected by the vagaries of the printing process and time. Also other models (and the real things, all of which will have been painted in colours that may or may not be a good match to the original and will change over time anyway.
Indeed the original paint chip is unlikely to be the same shade now as when it was painted, and I've read that the skill of the paint mixer was to create a shade that would be the correct colour after the loco had been steamed a few times and the heat had had its effect on the tone. Also, two paints that look identical under one light source, might look different under a different light. This is why it's usual; to compare colours against a neutral grey background under a balanced 'daylight' bulb.
For models to look realistic one of the main changes needed is normally to use a less glossy finish, in order to prevent the model looking toylike, but as models are smaller less saturated colours often look more realistic too. One reason for this is that the smaller model appears as if farther away, and things farther from us have their colours more muted.
So there are a whole host of psychological factors involved as well as the near impossibility of knowing what the actual colour was at the time.
I've fallen foul of this myself – questioning the colour of someone's 'improved green' LBSCR loco. I even found a photo of a real one to prove it. I had to eat humble pie when (a) it became clear that there are no two pictures of models or the real thing that appear the same colour and (b) the restored loco I had based my opinion on is notorious for being painted the 'wrong' colour.
My own little loco is far too deep a blue – the consequence of dulling the gloss with a satin lacquer that greatly deepened the shade. But the original is so faded, it's impossible to tell what shade of blue it was anyway 