Mixing Blue and Yellow Pigments
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.
Mixing Yellow and Black
That adding yellow to black can produce green is a mix most people discover by accident. It may seem improbable, but the combination produces an earthy, dark green. Again, different yellow pigments and different black pigments give different results.
Perylene black is a black pigment (PBk31) that's often labeled Perylene Green because it has a green undertone to it. Use straight from the tube, it's extremely dark, but spread it around or thin it with water/medium and you start to see the green in it. Mix with white and yellow, and it's very evident.
Adding a Blue to a Green
Never forget that you can tweak a green by adding blue to it. Again, different blue pigments will result in different greens. If you're painting a landscape, start by mixing in a little of the blue you've used for the sky rather than another blue. Not only will it give you a slightly different green to use, but it'll help the composition by creating a subtle color link between the greens and sky.
Landscape greens appear more blue or yellow depending on the time of day, and the angle of the sunlight. Adjust your greens accordingly. The most extreme is the short window of golden light near sunset that photographers love so much, where the sun throws a golden glow over a landscape.
Adding a Yellow to a Green
Similarly to tweaking a green by adding blue, so you should never forget the possibility of tweaking a green by a yellow. Not only the bright, intense yellows, but also earthy yellows such as golden ochre.
The greens in a hot landscape will lean more towards yellow than blue, so mix in a little bit of the yellow you've used for the sunny sky to create a range of greens.
Neutralizing a Green
Convenience Greens vs Single Pigment Greens
A convenience green is a ready-mixed green that you simply squeeze from the tube, created by the manufacturer from different pigments to save you the trouble of mixing it yourself. They're very useful for getting a consistent green, and the label will tell you exactly what pigments are in the color.
Two examples of convenience greens I often use are green gold and Hooker's green. What pigments are in these differs from manufacturer to manufacturer. For example, Golden's Hooker's Green contains anthraquinone blue, nickle azo yellow and quinacridone magenta (PB60, PY150, PR122) while Winsor & Newton's Galeria Hooker's Green contains copper phthalocyanine and diarylide yellow (PB15, PY83).
Obviously single pigment greens also come ready-to-use in tubes, but unlike convenience greens only contain one pigment. It's important to know which you're using if you're tweaking a tube green as the more pigments in a mix, the easier it is to muddy the mixture and the lower the chroma of the mixed color.