Most of that 6% don't have the useful form of tetrachromacy. Their response curve of their extra cone is almost identical to one of their existing ones. This is

Most of that 6% don't have the useful form of tetrachromacy. Their response curve of their extra cone is almost identical to one of their existing ones. This is like deuteranomaly (the most common form of colourblindness), in which people can't distinguish red and green because their L and M cones are too similar.