I checked out Blue and Yellow Don't Make Green by Michael Wilcox, which was recommended by   It is sooo interesting.

One of the questions he asks in the beginning is "What happens when you shine a red light on a blue colored object?".  My knee-jerk answer is purple .  In reality, you would get a gray/brown/blackish color because the blue colored object would absorb most of the red light, since blue objects absorb all other colors and reflect mainly blue color.

Through a series of watercolor exercises and a lot of explaining, he shows how blue and yellow do not make green.  Quite convincingly.  The basis of his argument is the fact that if you had a pure yellow pigment and a pure blue pigment (both of which don't exist as far as we know), the combination of those pigments would not reflect any color of light back because they would be absorbing each others' colors.  The reason why you can get green when you mix blue and yellow is because there is a little bit of green in each color and by mixing it, you are canceling out the blue and yellow and allowing the green to appear.  The book explains which types of colors to mix in order to get vibrant secondaries.

Another interesting point that he brings up is the importance of opacity when mixing colors.  Although watercolors are considered "transparent" for the most part, I have noticed that colors like Cadmium Yellow are rather opaque compared to other colors.  Opacity determines how light or dark the colored grays come out when you mix two complementary colors.  Mixing transparent complementary colors will make the grays dark, while mixing opaque complementary colors make the grays lighter.