There are 3 colours which have been documented to exist in gray foxes (Urocyon cinereoargenteus): the common grey colour, which is coloration found on the large majority of gray foxes; leucistic, which is beige with white areas; white, which is all white with a pink nose and brown eyes.
White & leucistic
Common grey & black
Gray foxes are generally pale to dark grey, with a white chest and belly, a black stripe going from the end of the back and down their tail and with tan, sometimes vibrant orange areas near their ears, chest, belly, paws and under their tail. Their noses are black but pink noses have, occasionally, been witnessed. Their eyes are dark brown or lighter brown. They are very commonly seen in the exotic pet trade, and quite often in the wild, as this is the typical colour which is described within the gray fox species, in almost every case.
Picture of Killua, shared with permission from Alex Madgalene (thecat_and_thefox on instasgram).
Black gray fox
Black gray foxes are just melanistic gray foxes. They appear to be completely black, with a black nose and dark brown eyes. Little information on them can be found, unfortunately. These foxes are rare and unheard of in the pet trade.