The very last glaze is applied to create the shadows and darkest tones within the leaves. Once again this is done using only a primary color, it's not glazed using a black. Remember to err on the side of caution, as it's far easier to add another glaze than to remove one.
Knowledge of color theory will tell you what color you need to use to produce the dark tone you want. The shadows in the leaves are complex tertiary colors (grays and browns) built up through the multiple layers of primary colors.
