What does a discolored tooth mean?

If the tooth is discolored from the inside, there is significant bruising of the highly sensitive pulp tissue inside the tooth.  If the entire tooth is discolored, the tooth is very likely to be non-vital because the swollen pulp tissue cannot expand with the hard casing of the tooth.  

In what cases can the condition be reversed? 

The cases that MAY still have a vital tooth tend to be either in very young dogs where the root of the tooth is still growing and there is room for the pulp to swell or if there is a very localized patch of
discoloration and the remaining healthy tissue may repopulate the injured area.