Why do plants look green?
The short answer
Chlorophyll absorbs red and blue light strongly and reflects more green light, so leaves look green to us.
How it works
The energy chlorophyll absorbs helps photosynthesis use carbon dioxide and water to make sugars. Green light is absorbed less strongly and is more likely to be reflected.
A familiar example
Leaves grown in shade can look paler because the plant adjusts its leaf structure and pigments to the available light.