Of the three primary colors, the three binary ones are formed. If you add to one of these the primary tone that is its opposite, it cancels it out. This means that you produce the required half-tone. Therefore, adding black is not adding a half-tone, it is soiling the tone whose true half-tone resides in this opposite me have just described. Hence the green shadows found in red. The heads of the two little peasants. The yellow one had purple shadows; the redder and more sanguine one had green ones.