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.
