Why are leaves green?

Because of the natural process of photosynthesis that always begins by absorbing light from the sun through proteins. And these reaction centres contain green chlorophyll pigments.

leaves appear green because the photosynthetic pigment chlorophyll is present in the leaves .. when sun light or any other light falls on leaves ,this chlorophyll absorbs all the other colours of light and only reflect green ..so our eye can only detect this green colour and it seems that leaves or chlorophyll is green but actually the matter is with absorption spectrum..
amazing fact :
if you fall red light on grass at night ,they will appear dark because they wont have any green light to reflect and they will absorb red light ..so our eye cant detect any colour :)