The standard idea is that the fourth type of cone is the anomalous type inherited from one&#x27;s father.  In the most common cases (deuteranomaly), it fills in between red and green.<p>In any event, while color perception is certainly subjective, both the underlying spectrum and the eye&#x27;s physiological response are very much measurable.<p>An interesting experiment would be to see whether she could construct two paints that she can tell apart but that look identical to everyone else.