The phrase "afternoon sky" refers to the sky during the time between noon and evening. It describes the appearance and quality of the sky during that specific time of day. Full definition
Other Stuff We Watched A Cure for Wellness Hacksaw Ridge No Highway in the Sky Lion Kedi Edge of Eternity 20th Century Women I Don't Feel At Home in This World Anymore The Bullet Train The Gate The Red Shoes Legion One of Our Aircraft is Missing Love in the Afternoon (filmjunk.com)
The mule train comes into the Washington area against a late afternoon sky in June of 1968. (npr.org)
This impressionistic landscape is bursting with vibrant autumn hues of orange, scarlet and gold against a clear blue afternoon sky. (ballarddesigns.com)