Why does this happen?
From the 'front' of the panel, the glass appears transparent. But viewed from the 'edges', it appears translucent...
Many of the comments have touched up on this, the light reaching the glass is experiencing internal refraction. The same reason that when fabric gets wet it seems to get darker. The same thing is happening inside the glass before reaching your eye.
This is what you are seeing in the glass but it would look someone more like this picture:
When the light hits the glass more of it is adsorbed into the impurities in the glass making it seem darker and greener. The glass (which has essentially been turned into a mirror now) is not as good at reflecting the other colors as green which is why putting a mirror next to another mirror makes that greenish hue that you see in the following picture: