One of the basic rules color theory is that blue mixed with yellow (or yellow with blue) produces green. And it's true. What needs emphasizing though is that the green you get depends not only on how much of each you use in the mix, the proportion of blue to yellow, but which blue pigment and which yellow pigment you use.
As painters we've many different blue and yellow pigments available to us, and each creates a different mixed green. Make a note of which pigments you're using so you can repeat the mix -- check the paint tube label for the color index number if you're using different brands of paint, don't rely on the name given to the color alone.
