The phrase "whipping a dead horse" means to attempt to revive a moot point or issue. "Whipping a pile of horse bones" means to keep making futile and pointless attempts to revive interest in something that no one has cared about for so long its rotted away into a skeleton of its former self.
Seasons 1 through 6 were the best. By season 8, FOX and Matt Groening were whipping a dead horse. Which means for the last 10 seasons they have been whipping a pile of horse bones.
Any specific type of thing that is different in a noticeable way, similar to a horse dyed with Jello powder so that is a different color. A new way of doing something.
In 1992 the Sony Minidisc was a horse of a different color when it came to sound recordings.
Well, that's the horse of a different color! <--In the Wizard of Oz when they refer to the purple and blue horses.