What is a fad color and why does it matter?
A fad color is a coat color disqualified by the FBDCA/AKC French Bulldog Breed Standard. These colors are also often referred to as rare colors. Those who intentionally breed disqualification (DQ) colors in French Bulldogs are motivated only to make money. No responsible breeder would breed against their breed standard.
FBDCA is bringing up the issue of fad colors in order to:

• educate and protect the dog-buying public,
• preserve our breed standard,
• protect the work of the many outstanding and responsible breeders who adhere to our breed standard.

A reputable and responsible Frenchie breeder should only breed dogs conforming to his or her country’s parent club French Bulldog Breed Standard. Those irresponsible breeders who breed for and advertise rare colors are intentionally spreading unacceptable color genes through the gene pool and causing problems for reputable breeders whose main concern is to produce quality puppies that conform to our Standard. These irresponsible breeders producing fad colors are motivated by greed, since by calling their disqualified colors rare they are able to sell them for very inflated prices.
For more in-depth information on this topic, please read the following:

• The AKC Breed Standard for French Bulldogs
• Intepretation/Clarification of Color
• A History of Color in French Bulldogs

Fad colors are not only undesirable for AKC-registered French Bulldogs. View breed standards and coat color statements by other kennel clubs, worldwide:

• Federation Cynologique Internationale (FCI) standard specifically states, “Disqualification: Colour of coat black and tan, mouse grey, brown.”
  Download the entire FCI French Bulldog Breed Standard here.
• Canadian Kennel Club standard specifically states, “Disqualifications:..black and white, black and tan, liver, mouse or solid black (black means without any trace of brindle)…”
  Download the entire CKC breed standard here.​

Please help maintain the integrity of the French Bulldog breed by choosing only Frenchies with coat colors as allowed by the AKC breed standard. Seeking and buying dogs with disqualifying coat colors only hurts the breed, even if you have no intentions of breeding an off-color Frenchie. Demand drives the market and if there is no demand for blue, chocolate, black & tan, or other unacceptably colored French Bulldogs, there will be no reason to breed these colors. Don’t financially reward people deliberately breeding Frenchies not accepted by our breed standard.

Another good site to read is
https://www.bullmarketfrogs.com/french-bulldog-breeder-warning-signs-part-one-fad-colors/