Because of dye lots, different manufacturers, time in the field, fabric lots and weaves and their level of dye acceptance, age etc. there are variations in color. Field grey shows up as a green toa grey to a brown with variations in between and tone values that are quite varied.
Photos are a great source of documentation but remember that they are also 60 years old, with various levels of under exposure, over exposure, color shift, development variations, fading, damage etc.
Extant examples of clothing will have much the same problems.
You can nit pick color formulas, paint lots, scale effect, weathering and still not come up with a definitive answer that you can unabashadly say that this or that color is the exact shade of fieldgrau that Landser Schmidt was wearing when he went trapsing about France bringing the message of National Socialism and visiting payback for the Treaty of Versaille.
Other than that....vallejo is good, as is tamiya, humbrol, mm etc....
I think if you go back to the At the Front website and look at the examples they actually have a side by side comparison of all the different shades. It's not something I'd worry about.
Mike
"Imagination is the dye that colors our lives"
Marcus Aurellius
A good friend will come and bail you out of jail...but, a true friend will be sitting next to you saying, "Damn...that was fun!"