One of the fundamentals of color theory or color mixing is that yellow and blue make green (see Color Class 1: Primary Colors). The theory is simple, but you can only learn to predict the results by practise, by actually mixing blues and yellows together, in various proportions and combinations. Exactly what green you get from the mix depends on what specific yellow and blue you used.
First I covered the canvas with the phthalocyanine blue. While it was still wet, I squeezed some cadmium yellow out of the tube, directly onto the canvas. Next I'd get vigorous with the brush, to mix the two together. How did I know how much yellow to squeeze out for the size of canvas I was working on? Experience from doing it before. As a rule, start with less rather than more, though if your green ends up too yellow, you can of course add more blue again.
