Comments on mixing paints, even after the old mix dried, are excellent. I, too, blend colors (craft paints -- oh my!). I have a color wheel, but I don't use it too much.
I mixed up a custom, brick-red color, for my Goblin army, which consists of 150+ minis. I have been able to re-mix that same, brick-red color, multiple times, over many years, dating back to the mid-1990's. I freely mix figures which were painted with different 'batches' of the custom brick-red color. And no one realizes that some figures were painted decades before the others, as they all look pretty uniform in shade/tint.
Like most things in this hobby, our skills improve with practice. I have even more Goblins to paint up, using the same custom brick-red skin color, and the previous figures were painted more than five years ago! I know I can mix up the same shade/tint, as needed. It is a skill, which improves with practice.
I am not a slave to any 'hobby color', such as snakebite leather. In the real world, leather colors vary by each lot, and each animal skin! I have taken a class on natural dyes for wool, which was hands-on. Natural dyes vary not only within different lots, but also as you cycle more batches of wool through the same dye pot batch! Each addition of wool absorbs dye from the liquid, making it less effective with each batch; each batch of wool absorbs dye at a different rate, apart from the dye batch, itself! Every animal's wool is unique, with individual properties for dyeing. You don't know how it will turn out, until you apply the mordant (color fixative -- each mordant chemical bath, has a different effect upon the organic dyes, altering them as the mordant does its work).
To say that only one particular shade of red/blue/azure/whatever, is historically correct… Is laughable, prior to the 20th century! The only method for achieving identical shades/tints, is to use synthetic dyes: synthetic dyes color using brute-force; they are perfectly consistent, when handled properly. Modern fabric dyers/makers, use synthetic dyes almost exclusively. They offer natural dye fabrics as a specialty offering, in limited amounts; they have disclaimers declaring that tints/shades vary, and this is normal, and they cannot guarantee anything.
Fortunately, I am a fantasy gamer, so consistent colors across an army, is not necessary for my troops. I do game with painted plastic Army Men figures, which I painted 15+ years ago. Even modern dyes fade at different rates, on every individual piece of clothing, so consistent colors are, again, unrealistic, unless they are in parade ground, brand new uniforms. ;-)
As a gamer, I understand the (unrealistic) desire to have consistent, uniform colors. I also understand how natural dyes work… To set the dye color, the wool/other material, must have a mordant added to it, to fix the dye, permanently setting the color. However, the mordant used, will alter the color, based on what it is, how strong it is, temperature, etc. For more information on mordants, see Wikipedia.
Precise, uniformly identical colors only really existed within paintings, thanks to the artists. Modern movies again reinforce this idea because they use synthetic dyes in their costumes, and they, too, like identical tints/shades in their wardrobes. Historical reality is swept away, and the masses don't even know it has happened… Cheers!
and I agreee entirely with the unrealistic protrayal of uniform colours.