The washes in the illustration below are the first layer. Colors used were carefully chosen with regards to spatial clarity and atmospheric perspective. "What!?!?!?!" all you landscape painters will complain. "How can you have atmospheric perspective in a matter of a few inches of depth?!?!?!" That is part of the illusion of botanical illustration.
To start this painting I used basically three colors: a yellow-green, a bright middle-green, and a grayed-up green. Some artists begin with a flat wash over the surfaces, but I like to start introducing form right away. The closest leaves begin with a pale yellow-green wash, preserving any white for highlights. The three front leaves are painted in the bright middle-green. The three back leaves are painted in the grayed-up green. This begins the illusion of atmospheric perspective. I'm afraid my scan does not show the true subtle differences in the colors.
yellow-green = hansa yellow + cerulean
middle-green = cerulean + hansa yellow
grayed-up green = cerulean + cadmium yellow