Coat a large saucepan with cooking spray; place over medium heat until hot.
Add carrot; saut 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.