Classic artists were very familiar with saponification. It happens every time you use oil paint. What you see is not necessarily what you get. The oil in the paint allows the chemicals in the pigment to move, blend, and chemically react with each other to form a new color. One year after you finish an oil painting, it will look very differently from the day you finished it. And they continue to change every year after that! A lot of people don’t want their paintings to change. I like knowing that my painting will continue to grow and change long after I’ve finished. It’s like they are alive.
And just because you blend colors of blue that appear identical when you mix one from cobalt blue and one from manganese blue does not make them identical chemically. They will appear drastically different after a year or even as early as a week later. Try painting something and take a picture of it freshly painted and hide it away for a year. Compare the old image with year old image. Hopefully you won’t be saying “What was I thinking?”