From the AKC Breed Standard:
All shades of red, sable and brindle. Black with or without tan or brindle points. Blue merle (black and gray; marbled) with or without tan or brindle points. There is no color preference. White flashings are usual on the neck (either in part or as a collar), chest, legs, muzzle, underparts, tip of tail and as a blaze on head. White on the head should not predominate and should never surround the eyes. Any color other than specified and/or body color predominantly white are disqualifications.
Blue eyes (including partially blue eyes), or one dark and one blue eye permissible in blue merles, and in any other coat color than blue merle are a disqualification.
Nose -black, except in blue merles where black noses are preferred but butterfly noses are tolerated. A nose other than solid black in any other color is a disqualification.
From the Illustrated Standard:
BLUE MERLE COLOR: CARDIGAN YES, PEMBROKE NO.
While the Cardigan Welsh Corgi and the Pembroke Welsh Corgi appear similar, they are in fact two distinct breeds that developed separately centuries ago in geographically distinct areas of Wales. They are part of the cultural heritage of Wales and each breed deserves to be preserved and protected like any cultural treasure.
One of the many breed characteristics that distinguish Cardigans from Pembrokes is color. Cardigans come in more colors including blue merle and brindle which have never occurred in purebred Pembrokes. Since canine geneticists have proven that both of these traits are dominant, it is virtually impossible for either color to suddenly appear in a litter of Pembrokes. Blue merle or brindle Pembrokes are a result of crossbreeding which is not condoned by the Cardigan Welsh Corgi Club of America, the Pembroke Welsh Corgi Club of America or their members. These puppies are not purebred, are not eligible to be registered in the AKC Stud Book and cannot compete in AKC Conformation or Herding events.
Through the action of recessive genes or through mating of a blue merle to other than a black alternate colors may be produced. These colors may be registered through AKC and shown in companion and performance events. They are a disqualification per the breed standard in conformation.
A complete list of colors and markings is available at the AKC Colors and Markings page. Choose “Cardigan Welsh Corgi” in the pull-down box.
CWCCA Color Studies and Reports (click to link to article)