In a large skillet, saute onion and the green and yellow peppers in oil until tender. Add garlic; cook 1 minute longer. Crumble turkey into skillet. Cook over medium heat until meat is no longer pink., Transfer to a 3-qt. slow cooker. Stir in the pumpkin, tomatoes, chili powder, salt and pepper. Cover and cook on low for 7-9 hours. If desired, serve with toppings.