 On 30 August 1954, he administered Extreme Unction to Ildefonso Schuster, who would be beatified in 1991.On 25 October 1960, Colombo was appointed Auxiliary Bishop of Milan and Titular Bishop of "Philippopolis in Arabia".He received his episcopal consecration on the following 7 December from Cardinal Giovanni Battista Montini, with Archbishop Anacleto Cazzaniga and Bishop Giuseppe Schiavini serving as co-consecrators.