Pink is a colour (rosa) and it is sometimes used to describe your cheeks – pink cheeks (sonrosados).
However, this article concerns “pink” as an idiom (modismo). Study these:
- Mr. Podemos´s Porsche is in the pink of condition (está en perfecto estado).
- Alejandro was tickled pink (estaba encantado) when Mrs. May became the British Prime Minister.
- The dish (el plato) was the pink of perfection (fue cocinado a la perfección).
If someone is “in the “pink” they are in good health.