Itten's color diagram, published in 1961, is still considered by many to be the cornerstone of color education. We show experimentally and theoretically that by mixing oil paints it is hardly possible to reproduce Itten's primary colors red, yellow and blue such that their mixtures produce Itten's secondary colors orange, green and purple. Optical models show why it is highly unlikely that paints can be created that follow the color mixing rules from Itten's color diagram. Our results confirm and explain earlier anecdotal evidence. We conclude that Itten's color diagram does not show how paint colors mix, and disagrees with optical theory and experimental evidence.