Outdoors you have two great sources of light, sunlight and sky light. The sunlight falls in parallel rays affecting everything in its path. Sky light which is weaker than sunlight, affects everything not directly affected by the sun. Sunlight is usually considered warmer than sky light and so shadows have a relatively bluish cast to all the colors within the shadow area when compared to those same colors in sunlight.
How does this affect green? The strength of the color of the sunlight shifts all of the colors including green. Sometimes greens appear olive or even orange to the eye even though we know them to be green. By observing the local color relationships of the scene we can see how the sunlight affects those local colors and key them accordingly. All the aspects of color change under these shifts not just the hue but also the chroma and value. To mix your green properly you have to paint the color as it appears not force the green hue into the key when it doesn’t belong there.
When I mix a color I always look at its relative components to the colors around it. I always start with its value and where its value fits in the painting as a whole. My next step is to determine its hue. When mixing a particular green I compare its hue to the other hues around it to determine how it relates in the spectrum. Is it more blue, red or yellow than surrounding hues?
Even if those surrounding hues are other greens, each green will appear slightly more blue, red or yellow than the others. If that difference is important enough for me to single out for its inclusion then I use it to help get me to the proper color note. The last thing I check my mix for is its proper chroma, its relative grayness to the colors around it. If all of these steps are completed properly I move one step closer to finishing the painting.