If a student asks you why we can see colors and you want to provide an answer based on the opponent-process theory of color vision, you tell the student it is because there are different types of cone receptors.

four                  

Explanation:

The answer is there are "four" different types of cone receptors.

          The concept of opponent process theory is first given by Ewald Hering. According to his theory, people can never see certain combination of colors.

           According to Hering's opponent-process theory of color vision, the ability to see colors is controlled by the receptor complexes that has opposing actions. They are the blue-yellow complex, red-green complex and black-white complex.

         The cells opposes the two color and we are able to see only one color at on  time. For example, we are able to see or perceive either red color or the color green at one time.

          We have four different cone cells or cone receptors in our eye and the condition of having four different cone receptors or or channels to perceive colors is called 'tetrachromacy'.

Therefore the answer is -

"four"

false, if this is a t or f question.