The color is due to retinal fatigue. You can see the same thing by staring at a bright red object against a white background. After about 30 seconds, you will start to see a green shadow or afterimage when you look at a blank part of the white background. Your retina has been desensitized to red, letting it register more of red's complementary color, green. The more time I spend looking at the red rocks around here, the greener the grey rocks look.