How does the visible light spectrum relate to photosynthesis?
Leaves absorb light to give their enzymes enough energy to perform photosynthesis. Two types of chemical are used to absorb the light, called chlorophyll and carotenoids.
As the graph shows, chlorophyll absorbs red and blue light, while carotenoids absorb blue and blue/green light. So the colour of leaves is light green, since that's the only colour not absorbed by the plant to perform photosynthesis.