Preheat oven to 375°.
Coat a large saucepan with cooking spray; place over medium heat until hot. Add carrot; sauté 5 minutes. Place flour in a small bowl. Gradually add milk, stirring well with a whisk; add to carrot. Cook over medium heat 8 minutes, stirring constantly. Reduce heat to low. Add Cheddar cheese and next 4 ingredients; stir until cheese melts. Add macaroni and red bell pepper; stir well.
Spoon mixture into a 2-quart baking dish coated with cooking spray; sprinkle with breadcrumbs. Bake at 375° for 30 minutes.