"I know color theory says that to get green I mix blue and yellow together, but I'm doing that and I'm not getting nice greens! What am I doing wrong?" -- Agatha
It's not that you're doing something wrong, but rather that when it comes to mixing greens, not all blues and yellows are equal. Some combinations produce more vibrant greens than others. Experiment with all the blues and yellows you've got, including using more than one blue and more than one yellow at a time.
Mix the green by adding a little blue to the yellow, rather than yellow to the blue. Why? It takes a lot less of a dark color (in this case blue) to change a light one (in this case yellow) than it does the other way round, so you'll save on paint.
If you're skeptical that you can mix spectacular greens and believe the only solution is to buy them in a tube, the greens shown in the painting here (see bigger version of painting) were created using just two colors -- Azo Yellow and Phthalocyanine Blue, from Graham & Co's range of acrylic paints. So try all the blues and yellows you've got, in different combinations and proportions, before you give up.
Remember to make a record of what you're doing -- what blue/yellow you're mixing and in what proportions -- so you can get the same green again next time!
Tip: Also try adding a little black to your yellows. This can produce surprising earthy greens.
See Also: Ways to Mix Green