Just a theory...
One reason that many visual observers of Mars may have seen areas as greenish may be that the rest of Mars is reddish. Staring at a particular color for a long time causes the eye to "fatigue" in an odd way--when you look at something else you get an afterimage of the complementary color. Looking at something red for a long time, like through a telescope, might cause the eye to begin to impart a greenish tinge to darker areas. Just a theory, and of course only applies to visual observations, not spectral analysis.