Mixing blue and yellow is the best-known way to mix a green, but it's by no means the only color recipe. This list of possibilities will help you expand your repertoire of greens, get you closer to that elusive "right" green, the one that Picasso was talking about when he said, "They will sell you thousands of greens: Veronese green and emerald green and cadmium green and any sort of green you like, but that particular green, never."
One of the basic rules of 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 also which blue pigment and which yellow pigment you use.
As painters, we have 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.
As well as exploring the greens you get from various combinations of blue and yellow pigments, don't forget about using glazing to produce an optically mixed green rather than a physical mix.
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. Used straight from the tube, it's extremely dark, but spread it around or thin it with water or medium and you will start to see the green in it. Mix it with white and yellow, and it's very evident.
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.
Similar to tweaking a green by adding blue, 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 toward 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.
If you've never added red or purple to a green, you're in for a pleasant surprise. Red or purple doesn't produce a vibrant green but rather works to neutralize it, to shift it more toward a brown-green or grey-green. Great for landscapes!
A convenience green is a ready-mixed green that you simply squeeze from the tube, created by a 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 we 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, contain only 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 is of the mixed color.
If you want to get seriously deep into the technical side of mixing greens, we recommend reading "Mixing Green" on the Handprint website. You will need to set aside some time to absorb it all, though, as it goes into great detail. Take an afternoon and pretend you're attending an art college lecture!