Why are plants green?

Plants are green because they have inside the chloroplasts (part of the vegetal cell) a pigment called chlorophyll.

The molecule of this pigment absorbes the magenta (red and blue) colors so the color of the plant (under white light) looks green (green and magenta lights make white light when they are mixed together).

The green pigment common to all photosynthetic cells, absorbs all wavelengths of visible light except green, which it reflects to be detected by our eyes