The germans used an index of industrial colors , RAL , with numbers identifying an exact color or tone. While recently trying to figure out how I was going to camo my schwimmwagen I asked my local paintmeister if had ever heard of the RAL color system. He said he had’nt but punched it into his computer , suprise , suprise , there it was with cross references for several deferent makes of paint and their particular toners. Dupont Centari industrial was particularly good because it could be flattened with a special hardener rather than a flattening agent which can effect the durability of the paint. So now when need to get that color right for that vehicle , field equipment or whatever you can do it correctly to code , as it ought to be
I you’re talking about painting models then colour doesn’t matter so much, as every vehicle would have been a bit different due to fading and battle damage as well as in field repairs. Though it is handy if restoring the real thing using the RAL system to produce the correct paint colour, though I had heard that reproducing colours using RAL isn’t exact in some cases as certain pigments and paints are banned, high lead and other chemicals?
I may be wrong of course 
Regards, Jack
You are correct. These colors varied quite a bit.
For the reasons you give and more.
Sometimes it varied with whatever was at hand to thin or mix the paste.
Clean or muddy water, gasoline, whatever was at hand.
The variations look good on models.