"…As I understand it, we of course agree perfectly about black in nature. Absolute black does not really exist. But like white, it is present in almost every color, and forms the endless variety of grays -- different in tone and strength. So that in nature one really sees nothing else but those tones or shades.
"There are but three fundamental colors -- red, yellow, and blue; 'composites' are orange, green, and purple. By adding black and some white one gets the endless varieties of grays – red gray, yellow-gray, blue-gray, green-gray, orange-gray, violet-gray.
"It is impossible to say, for instance, how many green-grays there are; there is an endless variety. But the whole chemistry of colors is not more complicated than those few simple rules. And having a clear notion of this is worth more than 70 different colors of paint -- because with those three principal colors and black and white, one can make more than 70 tones and varieties. The colorist is the person who knows at once how to analyze a color, when it sees it in nature, and can say, for instance: that green-grey is yellow with black and blue, etc. In other words, someone who knows how to find the grays of nature on their palette.
"… you will see when you come to the studio that besides the seeking for the outline I have, just like everyone else, a feeling for the power of color. And that I do not object to doing watercolors; but the foundation of them is the drawing, and then from the drawing many other branches beside the watercolor sprout forth, which will develop in me in time as in everybody who loves his work. …"
Extract from a letter by Vincent van Gogh written on 31 July 1882 to his brother, Theo.