there is a reason that you will be less likely to find any responsible breeder producing a rare color, show breeder or not. rare colors tend to be recessive and that means that not only will you be doubling up on those color genes to get what you want but all of the other nasty skeletons in the genetic closet. Most health problems are recessive. Allergies, skin issues, eye problems, even cancer risk will be increased.
Since the color is not common, you have less dogs to choose from for your breeding program. That means other issues have to be overlooked to insure that you get the color you want - the first to be bypassed is often temperament. Instead of breeding the best of the best dogs to produce the next generation, you have to use "almost as good" dogs simply because they have the color you want. The next generation you have the color a bit more common but you've already doubled up on all of the health issues as well. You'll have more inbreeding since so many dogs that share the recessive color gene will have common ancestors. Plus, the easiest way to cement a recessive trait into a bloodline is through close breeding of parent/child, brother/sister, etc
Agreed. Inbreeding usually leads to health problems.
Other times there is actually a reason. One example from my breed - GSD - is eye color. The standard calls for a dark eye. Why? Because of the job they were bred to do - a moving fence to control flocks of sheep. After all, border collies are required to have a light eye - again because of their job of herding sheep. Why the difference? The way they are intended to work. Light eyes are frightening to prey animals like sheep - they are predator eyes. The border collie moves sheep through rushing and stares. The dark eyed GSD is not alarming to sheep as he trots past on his herding duties. His dark eyes mark him as less threatening, not a prowling predator. When they have to separate a sheep from the herd, a BC rushes and drives the sheep. Crouching and staring and taking him where he wants to go. A GSD will drive the sheep, using his size if necessary to take the sheep where it needs to go. Form follows function. Why does the GSD have the angulation that he does? We'll avoid talking about the extreme look of some show dogs which is thankfully falling out of favor in exchange for more moderate looks. But it's so that he can move the way he does. The GSD "flying trot" is because of the way his body is put together. He is made to work for hours, running the perimeter of pastureland acting as a fence without stop. So, that long rectangular shape with a certain degree of angle to certain joints allow him to do that without his body breaking down despite long hours of work for 8,9, 10 years of his life (or longer).