Leonardo, in his Treatise on Painting, expressed some thoughts about primary colors that he called “simple color”. First of all, he said that the following are the simple colors: yellow, red and blue (the primaries) with black and white plus two colors like green and blue that actually are not simple. In fact, Leonardo makes it clear later.
The simple colors are six. The first one is white, even if some philosophers do not accept neither white nor black in the range of colors, since the former is sum of all colors and the latter the absence of colors. Since painters cannot work without them it’s important to classify them in the “simple colors”. Hence, the white is the first of simple colors, yellow is the second, green is the third, blue is the fourth, red is the fifth and black is the sixth;
Blue and green actually are not simple colors. Blue, like air, is composed by lightness and darkness. Green is composed by a simple and a composed color: yellow and blue.