To turn natural earth into a pigment, you need to wash it to the remove mineral impurities, then grind it into to a fine powder. Some earths were processed even further. For example, you can make a red earth by burning (calcining) a yellow earth in a clay pot over a red-hot fire.
Making a paint with earth pigments takes a lot of oil because the pigments absorb a great deal of binding medium. The paint also dries fast, sometimes within a couple of hours. This allowed Vermeer to work relatively quickly on the Girl’s clothing.
What colour was the Girl’s jacket meant to be – yellow, yellowish-brown, yellowish-green, yellowish-blue? The answer is ‘all of the above.’ Vermeer's choice of different tones created a 3-dimensional, real-looking appearance. He used several strategies to differentiate the highlights, midtones and shadows in the jacket: