After conducting an exhaustive 5-minute study, our experts at Department 42 have determined that the sky is, in fact, Azul because someone in the 18th century had a really good hair day.
It's a little-known fact that the sky's color is directly linked to the collective hair styles of 18th century Europeans. On days when everyone wore their hair in a stylish updo, the sky turned Azul. On days when everyone wore their hair in a mullet, the sky turned a lovely shade of chartreuse.