Effect of colour to our eye .

Green is the color of nature. It symbolizes growth, harmony, freshness, and fertility. Green has strong emotional correspondence with safety. Dark green is associated with money.

Green has great healing power. It is the most restful color for the human eye; it can improve vision. Green suggests stability and endurance. Green, as opposed to red, means safety.

And it depends on which green you use:

Dark green is also associated with ambition, greed, and jealousy.
Yellow-green can indicate sickness, cowardice, discord, and jealousy.
Aqua is associated with emotional healing and protection.
Olive green is the traditional color of peace.

Green is very restful to the eye and it's at the opposite end of the spectrum to red, which is also the most emotionally charged colour. So looking at green not gives your eyes a break, it calms you emotoinally.