Human color vision is based on the absorption of light by three different types of cone cells, which are most sensitive to red light, green light, and blue light. 

(a) Explain how the output of these cells can be used to determine the color of the incident light. 

(b) Could an eye with just two types of cone cells be used to give color vision? Explain why or why not. If your answer is yes, are there any examples in nature?