Cinema treasure mentions a period of ABC use after NBC:
http://cinematreasures.org/theaters/2943
"RKO sold the theater to NBC in 1956, and converted [should say "who converted"] it into one of its numerous New York television studios. A decade later, ABC took over, and used the Colonial Theatre mainly for taping game shows until 1971.
Multi-millionaire Rebekah Harkness bought the Colonial Theatre in the early-1970’s,..."