Why are plants green?

Parts of a plant that are green, such as leaves, are green because they contain chlorophyll.  Chlorophyll absorbs light energy (but less so green light than red or blue) and helps convert that energy into food for the plant as part of photosynthesis.

A more whimsical answer, which is not scientifically accurate, is that plants need both sunlight (which is yellow) and water (which is blue) to live and yellow plus blue makes green.