Go to this page and then come back. No this is not a trick or hallucination. There is a simple explanation. Without getting too technical, I'll tell you what just happened. In the back of your eye there is a thin layer of tissue that contains millions of tiny light-sensing nerve cells called rods and cones.
Cones respond to specific wavelengths of light.
Your eye is filled with color decoding cones. When you looked at the red box the cones that detect the red wavelength become tired and fatigued. When this happens the opposite cones in your eye start to kick in. Hence the bluish green color you saw. Now that you know there is a scientific reason behind eye fatigue you should apply it to your web site.