Green.
Why?
Green symbolizes nature. It is the easiest colour on the eye and can improve vision. It is a calming, refreshing colour. [Plus, it's the colour of the sin Envy, and that makes me feel evil =P]
Because blue is currently the most popular...
Did you know, it is recommended to wear blue to job interviews because it symbolizes loyalty? Blue also causes the body to produce calming chemicals, so it is often used in bedrooms.
I'll go on if people wanna know more ^.~
Edit: Woo I'm a Muckraker now!!