As someone who has studied military uniforms extensively as part of my hobby of wargaming with miniatures the precise colours attributed to particular uniforms at particular times in history can be hard to pin down even if you view them in museums.

It depends partly on how well the dyes could be fixed in the period in question and as importantly the environmental impact on them in service on campaign.

An example is the classic Napoleonic army blue which could be almost a dark purple in production but which would fade with rain sun etc so that in a body of soldiers supposedly wearing the same uniform some might well find their shades look substantially different to others. Indeed the assumption was that materials would fade to the proper colour at some point!

In the modern era that may be less problematic but simply because a thing is defined in a regulation or contract spec as a particular colour does not necessarily tell you the exact shade. And it depends how much effort is put into quality control. I have no doubt the British Guards regiments have had a great deal of effort made in ensuring the exact right shade of scarlet! But then they don't tend to get much environmental damage!

I have tremendous problems discerning khaki from faded khaki from olive green from drab in the acrylic paints that I use. So I can entirely sympathise with anyone trying to match colours on actual materials. Much easier if you're using mass produced contemporary officially produced items obviously. But even then batches may differ. I would not be surprised if even the finest plaids produced 50 60 or 70 years ago have partially faded or not quite so well survived regular out of doors use.