I've encountered this a lot since getting Nami - who is very obviously not a white-coated Kishu Ken: "white is the preferred color in the Kishu Ken!", "I thought Kishu Ken were only white..?"
Let me explain to you a thing: no, it's not and no, they aren't.
When people acknowledge the other coat colors, it's usually in the same way one acknowledges an extinct or critically endangered animal: "they used to happen." They, quite clearly, still happen and there are Kishu breeders (not many) whose kennels are dedicated to preserving the non-white (yushoku) Kishu Ken.
It's actually pretty hurtful to hear such things spread around. Not just as an owner of a non-white dog, but as an enthusiast of the breed in general. To continue to insist that white is the preference means that a culture is being built. A culture that declares that these non-white dogs are not worth showing, are not worth breeding, and are intrinsically inferior. That harms the breed by cutting out valuable genetic information and diminishing the already-compromised genetic diversity of the breed. This does not only apply to the Kishu Ken. There are plenty of other breeds where colors are arbitrarily avoided or selected against because they do not win dog shows, even when those colors are perfectly acceptable per the standard. I just can't speak for those breeds.
The unbiased, factual truth is that there is no preferred coat color written into the standard among the accepted coat colors (that is to say, as long as the coat color is not explicitly disqualified - as is the case with pinto and brindle coats) in the Kishu Ken. The reason you see more white dogs is because of the "popular sire effect". The white gene is recessive so both parents must have a copy of the gene to pass it on - and we know exactly where its prevalence came from in the Kishu Ken: extensive breeding toward a particular line of Kishu Ken that was popular at the time. These dogs were bred upon and inbred on because they had traits that were desired by breeders, exhibitors, and/or hunters. It was not just for the coat color.
To this day, white remains prevalent for a few reasons: the way the white gene is inherited means that you can ONLY get white puppies when you breed a white dog to another white dog, but if you breed two yushoku dogs, you could get any color - including white. The gene is spread so far through the population that it takes care to breed against it, now. As such, the most competitive individuals of the breed are naturally going to be available in white.
Breeding and showing a white dog is easier in the Kishu Ken. There's no pesky additional coat color rules to pay attention to and there's far more stock to select from. A white Kishu Ken wins shows not because white is the preferred color, but because there are more examples of white dogs that fit or exemplify the standard in other ways. It is simply highly unlikely that a yushoku individual will be able to compete with a white-coated dog. Additionally, there are a variety of coat color faults that show up on a yushoku dog that do not show up on a white dog. The white coat itself is usually to blame for these coat color faults that show up in the Kishu Ken. Usually, the red pigment on a yushoku dog is too washed-out or faded - in the Japanese breeds, the Kishu Ken is a cautionary tale of why one does not want to breed for cream/white because of this. A strong, red pigment is a major aspect of correct coat color. Sometimes, yushoku dogs have poor sesame - the sesame pattern is too sparse or too muddy for some reason. Many times, yushoku dogs also have black masks, which are not desirable. Proper urajiro also appears rare in the Kishu Ken - another no-no in the Japanese breeds (and another reason the other breeds tend to avoid white coats).
It seems as though it is much more difficult to breed and successfully show a yushoku dog. Saying that the white coat is preferred is like crippling your opponent and then saying that you were always the favorite to win, anyway. The game is more difficult for the yushoku dog because all of the worthwhile resources were taken up by the white dog near the start of the game.
I actually really dislike talking about this "issue" because it is a silly, aesthetic thing to get caught up on. I don't like the Kishu Ken - personally - because they are blue with purple polka-dots. I like them for the many, many other traits they have that make them the perfect companion for my life. The impact this issue has, however, is very real and potentially detrimental to the breed as a whole. I could not call myself an enthusiast if I encouraged arbitrarily devaluing and cutting out healthy dogs that can offer valuable genetic diversity to an already small genepool because of a coat color.
If you're interested, please visit the Yushoku Kishu page!