But that is just it, you have never in any experiment shown that a third cone has ever developed. You "assume" that is the case, yet mutational research has shown genes can be lost, but never once has any shown genes can be gained. Had even one shown such you might have an argument, but being they have only shown such can be lost (fish living in caves losing eyes), mutations causing damage and the gene rejected, etc, there is more evidence for assuming dogs lost a cone.
Humans already have a 4th in a percentage of the population, so 3 cones being the norm is most likely due to loss of genetic information through mutation, since most mutations are neutral or recessive. Those that are positive merely recombine what already existed. And in plant research most positive mutations are considered positive because a gene went recessive that made the plant not fit for human consumption. Not positive because it gave the plant a feature that enabled it to better survive.
Sure, a seedless orange is better for us, but not for the orange tree, being it is no longer able to propagate without direct help from mankind. Besides which that was done through genetic recombination and grafting, not mutation.
I am not denying your assumptions are not "hypothetically" possible, but since it has never been observed in any genetic experiment ever done, you are asking me to accept as fact something contrary to the evidence. Being born with blond hair instead of brown is not evolution of a new hair type, since all hair types already exist within the genome. It is merely variation of what already exists.
In the end, this is what every experiment has shown, yet you conclude evolution is through mutations creating new alleles and genes, something not once observed.