Doug yes. That is why professional painters try to mix together the paint they have, even off the shelf paint, because there will be variations with each batch. Or, they will paint one side out of one can, and another side out of another can and then mix the left overs from the 2 cans together. The eye doesn’t pick up slight variations as well, if they are 2 perpendicular surfaces.
The real problem is that matching paint from photos is almost impossible, as the type of film and the age of the photo will vary all over the map. Also, when trying to match a color to a prototype, using an actual chip of paint off that prototype will render wildly different hues of the basic color. Another variable is that paint color as perceived on the 1:1 item is seen by your eye in a different way due to the light on the given day. Model paint companies try their best to match the prototype colors. Poly S was pretty good at this with the colors names were fairly descriptive. It is a quagmire of variables, as paint when wet is a different shade from the dry color.
Another variable is what the paint company specializes in as far as the intended surface to be painted. Vallejo and Humbrol are big in Aircraft and miniature soldier colors. Tamiya and Testors are heavy into the military vehicle area as well as a huge line of Aircraft colors. Even the difference between Flo-Quil and Poly S (the same company) of the same name are noticeable because one has an Acrylic base and the other is Enamel.
My rule of thumb is to look for a paint that looks right to your eye judging from, either prototype observation, or several different newer photographs of the prototype. If you are really into exacting authenticity, you can sometimes contact the particular railroad you are modeling (If it still exists) and ask them for their color mix numbers.
Yeah all of what you said is true plus the prototype itself changes as paint fades and weathers and a new batch is applied. My particular prototype I have seen change shade over the years. So I just “know” what it is supposed to look like and if you don’t like it paint your own
Andrew, yes, and that was what I was trying to say earlier, but you have explained it so much better. I think the online pictures of my chosen subject are Kodachrome slides. And anyone with even a casual relationship with photography knows that Kodachrome accentuated some colours, some way beyond what they should have been accentuated.