When you're in the pink, you're extremely healthy. If you tend to worry about your pet rabbit, you'll be glad to have the vet check him out and say he's in the pink.
While the old-fashioned phrase in the pink brings to mind the pink, healthy cheeks of a child playing outdoors, it started out with a slightly different meaning. Apparently coined by Shakespeare, in the pink was originally the peak or culmination of something — "in the pink of the mode" was the height of fashion, for example. Eventually, it came to be specifically associated with the pinnacle of good health.