Humans are trichromats which means that our eyes have three types of cone cells which are sensitive to different parts of the spectrum: red, green and blue light. They perform a non-unique mapping from an arbitrary spectrum of light into three signals which are known as a tristimulus which we perceive as a particular color. Some animals are dichromats and some are tetrachromats, and some humans have defective cones which is the cause of color blindness.