PINK. "Highest form or degree. Prime." A runner in the pink of condition is at his/her prime.
BLUE. "Depressed, dejected, melancholy. Holding or offering little hope; bleak, dismal" "Somewhat obscene, risque"
So being in the pink is a good thing. Being blue is not so hot. How did blue become associated with boys and pink become associated with girls? Boys are not supposed to like pink? Girls are supposed to avoid blue?