Oregon and UCLA went into Wednesday's action tied for the lead in the Pacific-12 Conference and both lost. The Bruins fell to Washington State 73-61.
Oregon will finish the regular season Saturday at Utah while UCLA will visit Washington.
Johnson hit all of his seven shots from the field and scored 22 points. Colorado took a 37-21 halftime lead and coasted to its fourth win this season over a team ranked in the Top 25.
Spencer Dinwiddie added 17 points and seven rebounds while Askia Booker had 11 points for Colorado (20-9, 10-7 Pac-12).
Oregon shot a mere 37 percent from the field and went 2-of-12 from 3-point range. The Ducks built an 18-2 record through late January, but have gone just 5-5 since then.
Arsalan Kazemi, Johnathan Loyd and Ben Carter all produced 10 points for Oregon (23-7, 12-5).