In the theory of Five Buddha Families, green light is associated with Karma-family, which is where Green Tara comes from.
The family of Karma-Buddhas, green, in its neurotic aspect stands for a tendency to see imperfections everywhere, to get stuck in analysis-paralysis, to overestimate one's game skills while staying on the sidelines, to feel like one is missing out or falling behind on all the myriad things going on in the world. In its enlightened aspect the same energy manifests as all-accomplishing compassionate action: clearly seeing what needs to be done, and doing it on the spot with no hesitation, without worrying about other things one could do.
As Chogyam Trunpa explained, the point of generation-stage meditation is not as much to visualize the deity's external form, as it is to generate the mood of the deity. In this sense, green color would stand for the mood of compassionate action, either healing/protection or destroying, depending on whether it is the peaceful or the wrathful form that we're generating. Because Tara is a peaceful form, in this case green would represent the soothing energy of protecting from danger and repairing the damage, similar to what a good nurse projects at a panicking patient.