"The wall is pink."
Translation:Ο τοίχος είναι ροζ.
Here we have: " Ο τοίχος" which refers to the walls of a building or other structure. It is masculine thus Ο.
"Το τείχος" which mean the walls used for protection from the enemy, floods etc. It is neuter thus το.
E. g. Τα Βυζαντινά τείχη της Θεσσαλονίκης (here it is plural) "The Byzantine Walls of Thessaloniki"