According to the trichromatic theory of color vision, people who have red-green color blindness have:

Answer;
According to the trichromatic theory of color vision, people who have red-green color blindness have normal blue-sensitive cones, but their other cones are either red-sensitive or green-sensitive. therefore, red and green look the same to them

Explanation; 
According to the trichromatic theory of color vision, there are three varieties of cones. Each type of cone is especially sensitive to certain wavelengths;- red light, green light and blue light. Thus, a given cone will be very sensitive to one of the three colors and only slightly responsive to the other two.
This theory provides a good explanation for the most common form of color blindness; that is red-green color blindness. People with red-green color blindness cannot discriminate between red and green. This is because they have normal blue-sensitive cones, but their other cones are either red-sensitive or green-sensitive.