Okay this is a long answer so i'll try and cut it down:
1) the pigment in chlorophyll is green (in most plants)
2) the colour of the pigment determines the wavelength which can be absorbed.
3) those with green leaves can absorb all colours EXCEPT green, which is reflected back hence why we see green
4) I would assume that most leaves are green as it allows more wavelengths of light to be absorbed, so photosynthesis can occur at a faster rate
Hope this helps,
Have a good day :)