h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

...Excellent Entry except for one pedantic point. You so almost answered the question, but you just tail off. As it stands the child would probably say 'but why is it green?' after you told them. The reason that the grass is green is because that it traps all the light (as well explained) except the green bits, which is reflected into your eye thus giving the grass its colour Well, enough pedantry from me, until later....
BCNU - Crescent

This still doesn't explain why plants have a pigment that rejects the prevalent frequency of sunlight.

I have heard the explanation, which goes something like this: early plants evolved in competition with pre-existing organisms (bacteria?) that contained pigments that did absorb green light, so plants evolved pigments that absorbed the light left after the green light had been removed.

Dry.