20 years ago I worked at a shop where they mixed PPG paint for the body shops in the area. It was a "recipe" based on a scale. A can of "neutral paint" would weigh XX grams or whatever, and then you'd add xx grams of Color 1, xx grams of Color 2, etc till you hit the total. They had to do it in the order the cards stated. I never did it, the owner did, they did it all by hand. I saw all sorts of colors mixed into "white" or "black" or "blue" to get the recipe.
I'm sure there were tons of errors in the process, but I never recall a can coming back.
I certainly hope it's a bit more scientific now.
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Rosewood 1983 911 SC Targa | Black 1990 944 S2 | White 1980 BMW R65 | Past: Crystal 1986 944 na
Guards Red is for the Unoriginal