After years of painting, I conclude that it takes really long time to see.
Moreover, it takes even more time to mix that color.
This is why I usually paint a small color study—from a live model—before embarking on a bigger painting.
And, what we often fail to understand about painting is its chemistry.
Paint is made of all different parts of earth: soil, minerals, precious metals, oil, etc.
To paint seriously means one also needs to have a thorough understanding of chemistry of colors, painting surfaces, and time.