From Wikibooks

What is photosynthesis?

Photosynthesis is making sugar using the energy of light.

Why are plants green?

Plants are green because they have green Chloroplasts.

But why are Chloroplasts green? Chloroplasts are green because they contain the green pigment chlorophyll in their thylakoid membranes. Chlorophyll is a pigment that absorbs red and blue light.