Below are some drawings of many (far from all) of the colors and patterns that are genetically possible in Great Danes. Note that the breedings that would produce many of these colors/patterns are NOT recommended by the GDCA.
Although some very experienced breeders (trying to improve a particular structural trait or health) may occasionally cross-color breed, and thus may occassionally have unusual colors, such a breeder will sell these pups on AKC Limited Registration, with a spay/neuter contract.
Some people sell these colors as "rare" (often for a very high price). As can be seen from the preceeding infomation on how coat color genetics works, and the GDCA Color Code, these "rare" colors are simply the result of breeding away from the GDCA color code. Such a Dane is NOT worth more than a "regular" color, and, any breeder attempting to sell these puppies as rare and valuable is NOT breeding for the betterment of the breed!