In a letter to his brother Theo, dated July 1882, found in Ever Yours: The Essential Letters, Vincent van Gogh describes how the simple few fundamentals combine into nearly endless permutations.
Absolute black doesn’t in fact occur. Like white, however, it’s present in almost every colour and forms the endless variety of greys — distinct in tone and strength. So that in nature one in fact sees nothing but these tones or strengths.
The 3 fundamental colours are red, yellow, blue, “composite” orange, green, purple.
From these are obtained the endless variations of grey by adding black and some white — red-grey, yellow-grey, blue-grey, green-grey, orange-grey, violet-grey.
It’s impossible to say how many different green-greys there are for example — the variation is infinite.
But the whole chemistry of colours is no more complicated than those simple few fundamentals. And a good understanding of them is worth more than 70 different shades of paint — given that more than 70 tones and strengths can be made with the 3 primary colours and white and black. The colourist is he who on seeing a colour in nature is able to analyze it coolly and say, for example, that green-grey is yellow with black and almost no blue, &c. In short, knowing how to make up the greys of nature on the palette.