I was hoping to get the answer for this myself but i guess im not alone. im no expert on this but my guess is it might have to do with the seasons. fire/red(black for ash) for summer heat, earth/green for spring, water/blue/white for winter, and air/yellow/red for fall to represent the leaves and it tends to be breezy in the fall and in Korra when she was first learning air bending, they did say to be like the leaf, go with the flow and change directions, like how fall is change in season and the change of the color of the leaves is a big representation of fall, also leaves blow in the wind.
or i could be totally wrong. saw others mention earth being green seems not appropriate so if i am wrong with my season idea then im at a loss for air (unless its cause buddhist monks wear orange/yellow/red) but if we go in a diff idea from the seasons, then my explanation for why earth is green might be because when you look at a picture of earth you see mostly white/clouds, blue/water, and green/brown/land/earth. brown is too neatral, also i'm certain if you were to ask an ordinary person (especially kids) to draw a basic picture of earth, i bet it would just be blue for water and green for land. also they do tend to have those crystals in the show that have the green glow. (and now that i think about that, king Bumi seemed to be the only one that ever showed any kind of bending of any crystal...i could be remembering wrong though) sorry for the rambling lol