That&#x27;s not quite correct. Where one cone type has a peak response, another may respond with a signal that is only one percent or less of its peak response, but the frequency regions in which hey have _any_ response basically overlap. Given the sensitivity curves, that difference is academic for blue versus red and green sensitive cones, but not quite for red versus green sensitive cones.<p>What differs is the amount in which having a fourth type of come allows one to get rid of metameries (<a href="https:&#x2F;&#x2F;en.m.wikipedia.org&#x2F;wiki&#x2F;Metamerism_(color)" rel="nofollow">https:&#x2F;&#x2F;en.m.wikipedia.org&#x2F;wiki&#x2F;Metamerism_(color)</a>, a term that the article surprisingly doesn&#x27;t mention)<p>If your fourth come type has a response curve that is very similar to one of te &#x27;normal&#x27; three, there should still be _some_ effect, but it will be hard to devise an experiment that shows the ability to discriminate additional colors.<p>But given the impact that not having red or green cones with their fairly similar sensitivity curves has, I suspect having a fourth curve, even if it falls between the two, will have a measurable (in the lab) effect on one&#x27;s ability to discriminate colors.