Green light moving erratically over Stapleton at 3 am

On my way to work at 3 am I observed a bright green light moving erratically in the sky over the Stapleton neighborhood near the old airport tower.  The speed at which it moved and change direction indicated that it was not an airplane or helicopter.  It was moving north to south then dipped quickly out of sight.  It made no noise and reappeared moving slowly, the bright green light turned to red then to a white light, it then quickly dipped down again out of sight.  I have a science background and am very analytical, but I can't explain what it was that I was seeing.