I think what others may be hinting at is that not all of the co-dom or dom individuals acctually make it to sexual maturity, they may be produced by random mutation which is fairly common though many don't even make it out of the egg before being killed. Random mutations that affect color morph may be created by a gene mutation that terminates the development of an intact renal system (just an example). One gene doesn't necessarily affect only one thing like color, it works on many different things from size, to shapes, to genetic fitness to heart murors to vision problems along with the color, etc etc.
Hope that helps
~mike