The following includes many excerpts from the article “Musings on Color” published on the blogspot Musings of a Biologist and Dog Lover written by Stephanie.
I’ve added my own thoughts and comments to embellish and round out the information for the Gordon Setter lover and breeder.

Musings of a Biologist and Dog Lover
Sunday, July 1, 2012
The Gordon setter is one of a small number of setter breeds, which also includes the English setter, Irish setter, and Irish red and white setter. Though the Gordon Setter now only comes in one acceptable color, the breed’s history included a number of other colors that are now considered to be mismarks. Part of why these colors are in the breed is due to its relationship with the other setters. So, what are these mismarks?
Dogs with more or less than than required in the standard
*Based on the current breed standard the Gordon Setters depicted in this artwork from the 1940’s carry too much tan.
Inherited on the Brown locus, a dog must be bb to be liver

Inherited on the Extension locus, a dog must be ee to be recessive red

Inherited on the Spotting locus, a dog with a variety of genotypes can have too much white
Looking at these mismarks, they are all recessively inherited except in dogs that are genetically solid but have too much residual white. All of theses colors were well known when the breed was young. Much like the Irish setter, the predominant color in the early years of the breed is actually not what you think it would be when looking at modern dogs. Gordons were once mostly tricolor with some dogs being solid black and tan, liver, or red, but the white markings and other colors fell out of favor and led to the production of the breed you see today.
The current breed standard for the Gordon only allows for black and tan dogs with specific tan markings. Dogs that are anything other than black and tan are disqualified and anything more than a small bit of white on the chest is not allowed. A dog with more or less than the required tan would be penalized, despite the fact that tan markings can vary greatly on dogs that are all genetically tan pointed. So far, it is known that there are modifiers that control this amount of tan, but it isn’t known where they are or how they are inherited.
This is a case where color standard is based on, basically, fashion. What once was popular was no longer liked by those who wrote the breed standard, and thus those other colors faded into obscurity. However, since the colors are basically all recessively inherited, they continue to pop up on occasion in litters that are born today. These past decisions are really problematic when looking at the breed’s history and what this holds for the future. Stephanie
Sally says…It is at this point that as a breeder of Gordon setters I would step in to say that I disagree with Stephanie’s call that the Gordon’s color standard (black and tan) is based on fashion and that this preference is problematic for the breed.
The color preference written into the standard was developed well over a century ago by avid bird dog breeders. They didn’t have an eye to fashion when it came to writing their standard, but they did know exactly what kind of dog they wanted to hunt over, as well as why those traits, written into the standard, were important to them. It has been my understanding that the vivid black and tan coloring of the Gordon Setter may have been written into the standard as the preferred color because of it’s contrast to the gold, tan and red foliage of the fall hunting season, the black dog contrasting, standing out against the fall foliage, making it far easier for the hunter to follow the dog while he worked the field. A red or buff dog, even a white and tan dog would more easily blend into the foliage, his coat acting to camouflage him as he worked the field. And, with the deaf link to the white gene it is certainly understandable why white was weeded out as an allowed color.
If there are other history buffs out there who can offer more insight as to the color preference we’d love to hear your thoughts and hope you’ll share those and any references for us in the comment section of this article.
We’d also welcome your photos, if you have some to share, of red, liver, or other mismarked Gordons to be shared here to help others learn. Please email those to us at gordonsetterexpert@gmail.com, we will respect your privacy and photos can be published anonymously.
Charlie Royster was kind enough to share this example of Gordon Setter color photo with us
Sally Gift, Mesa AZ