Among the possible colors for this breed, the solid black Pug dog is very popular. A true black Pug will be that color depending on the genes of the parents.
There are 3 possible types of black coats:
The first is a solid black, the dog will have no other colors at all on the body (no markings), Good breeding practices and good health will lend for a very glossy and shiny coat.
The 2nd type will have small patch of a secondary color...if this is the case, that 2nd color is know as a mismark (In show, this is not a disqualification, however it is a fault)
The 3rd type will be a blended coat. Other colors may be a dark tan or red and the way in which the coloring is blended together often is in the brindling pattern (stripes). This type of color patterning is not accepted by the AKC (there are some other dog breeds can be registered as Brindles), but not the Pug).
Therefore this and the mismark would all be categorized as black or fawn depending on the base color, because if a person is registering a Pug and there is no option to check off coloring that accurately describes the dog, one must choose the color that comes the closest. This is why some brindled or parti (2 colors) Pugs are registered as black.