Continue mixing the yellow and blue until you've got the green you want, over the area you want. Add more yellow if the green is too dark or blue, and more blue if it's too yellow a green. How uniform a green you get depends on how thoroughly you mix the colors on the canvas, just as it does when you mix colors on a palette.
What I like about mixing colors directly on a canvas is that I can get variations in the mixed color. So, in this example, I've mixed pieces of green that are darker or lighter in some parts of the canvas than others. I can also blend the green into the area at the top that I'll be leaving as blue, for sky in the background.
Don't confuse this color mixing technique with glazing. Mixing colors directly on the canvas is quite different from painting glazes, where you're creating an optical mixture rather than a physical mixture.
