Mask

Here, 'mask' means an eumelanin masking, which can range from covering the tip of the muzzle to most of the head. It is a separate gene that can appear on any pattern. However, the mask will naturally not be visible on f.ex. a recessive red dog. If a dog is red or cream with a mask, you can be certain it is actually a clear sable you are looking at. Likewise, you cannot see a black mask on a black dog, though on a tanpointed dog, the mask will be visible be visible as incomplete facial markings. A heavy mask may cover up all or most of the tan on the head on such a dog.
Interestingly, if the dog also has the greying gene, the mask will not grey with the rest of the dog. That's how you can get a blue kerry blue terrier with a black face

Masks are eumelanin (on a phaeomelanin background) pigment so they can be black, liver, blue or isabella, of course matching the rest of the dog's pigment. 

German shepherd dog (sable) with a black mask

Catahoula leopard dog (blue merle) showing a mask in the tanpoint pattern.