The Wildcats advanced to face No. 10 Texas A&M on Friday in the quarterfinals in an early tournament matchup of ranked foes.
Brittany Spears led Colorado (11-18) with 23 points and Kara Richards added 22, but they were the Buffaloes' only viable offensive threats. Only three other players scored as the Buffs lost their sixth straight game.
Colorado has won only one Big 12 tournament game since 2004.
It was Kansas State's second win over Colorado in less than a week, completing a three-game season sweep of the Buffaloes.