Two 'secrets' to successfully building up colors by glazing with watercolors are to select primary colors that have only one pigment in them, and to be patient enough to allow each glaze to dry completely before painting the next.
The leaves were painted by botanical and zoological artist Katie Lee, who kindly agreed to my using her photos for this article. Katie uses a six primary palette, comprising a warm and cold blue, yellow, and red (see: Color Theory: Warm and Cool Colors). Her paper of preference is Fabriano 300gsm hot pressed, which is a thick and very smooth watercolor paper (see: Weight of Watercolor Paper and Different Watercolor Paper Surfaces).
The other essential to successful glazing is a thorough knowledge of what results you're going to get when you glaze a color on top of another, how the colors interact with one another. It's something that can only be acquired by hand's on practice until you're internalized the knowledge and it becomes instinctive. (Exactly how is beyond the scope of this article, but basically paint samples, keeping careful notes of what colors you've used.)
This photo shows the initial glaze, and at this stage it's hard to believe that the leaves are going to turn out as beautiful greens. But the choice of initial glaze isn't arbitrary: it's yellow in those parts of the leaves that will ultimately be a the 'brightest' green (warm green), blue in those parts that will ultimately be a 'shadow' (cool green), and red in those parts that will be brown.
Isn't is amazing what a difference a layer of paint can make? This photo shows the result of one glaze over the initial glaze, and already you can see the greens emerging. Once again, only blue, yellow, or red has been used.
Remember, if a layer of paint needs to be totally dry before you glaze over it. If it isn't totally dry, the new glaze will merge and mix with it, ruining the effect.
This photo shows what the leaves look like after a third and then a fourth round of glazing was done. It really does show how glazing produces colors with a depth and complexity that physical mixing of colors simply doesn't produce.
If you want to lighten a section, such as a leaf vein, you can lift off watercolor even if it's dried (see How To Remove Errors in a Watercolor Painting). Use a thin, stiff brush to do it, but avoid scrubbing the paper or you'll damage the fibers. Rather leave the paint to dry then lift off some more.
Once you've got the main colors working to your satisfaction, it's time to add the fine details. For example, where the edge of the leaf is turning brown and the leaf veins.
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.