First, you need to differentiate between "pink" as in skin color, and "pink" as in a rash.
Every coyote I catch in Ohio has a pink belly. But not every one has mange (although plenty do). Generally, these coyotes show a pink belly because they are not well furred underneath, and thus not the best specimens of coyote you can find. I would not be surprised if you get a lower price for these coyotes as opposed to those that are fully furred.
There is really nothing you can do to change the skin color of a coyote, this would not increase the fur density anyway. And, likewise, there is no way to add fur to a coyote after it is dead.