Is it reprehensible for a man to cover his head with black in prayer or outside of it?

There is no dislike (karahah) in covering the head with a black turban or a cap during prayer or otherwise. Indeed, the Prophet, may Allah bless him and grant him peace, entered Mecca wearing a black turban, and Atā’ bin Yazid al-Laythi prayed with a black turban wrapped around his head.