Fickell had been noncommittal about his starting quarterback after the team's 24-6 loss to Miami last weekend, and said Miller and Joe Bauserman would compete in practice this week.
Bauserman started Ohio State's first three games, completing 30 of 60 pass attempts for 365 yards with four touchdowns and no interceptions.
But the senior struggled mightily in the loss to Miami (1-1), completing just 2 of 14 passes for 13 yards. Miller was 2 of 4 passing against the Hurricanes with an interception and a lost fumble.
Miller will be the first freshman quarterback to start for the Buckeyes (2-1) since Terrelle Pryor in 2008. Pryor also started his first game in Week 4.
Miller, who sat out against Toledo in Week 2 with a minor injury, completed 8 of 12 passes for 130 yards and a touchdown in the season opener Sept. 3 against Akron.
Colorado is 1-2.
Adam Rittenberg covers Big Ten football for ESPN.com. He can be reached at firstname.lastname@example.org.