Found this at Mirabilis---
"Dr. Neitz, who conducts his research with his wife Maureen, said only women have the potential for super color vision.
That's because the genes for the pigments in green and red cones lie on the X chromosome, and only women have two X chromosomes, creating the opportunity for one type of red cone to be activated on one X chromosome and the other type of red cone on the other one. In a few cases, women may have two distinct green cones on either X chromosome.
But it's unlikely, Dr. Neitz said, that all of the women with four types of color cones will have the potential for superior color vision, because for many, their two red cones will be so close to each other in the wavelengths they detect that they won't see things much differently than a three-color person does.
He estimated that 2 percent to 3 percent of the world's women may have the kind of fourth cone that lies smack between the standard red and green cones, which could give them a colossal range."
Interesting but it gets me not closer to knowing why my husband can NOT tell when I get my hair cut. It does explain other things...like knowing what ties go with what shirts.