While pasta is cooking, melt butter in 3-quart saucepan over low heat. Cook garlic in butter 30 seconds, stirring frequently. Stir in flour and salt. Cook over medium heat, stirring constantly, until mixture is smooth and bubbly. Gradually stir in milk. Heat to boiling, stirring constantly. Boil and stir 1 minute. Stir in mustard and Colby cheese. Cook, stirring occasionally, until cheese is melted.