Heat the oil in a large skillet over medium heat and saute the onion, green bell pepper, yellow bell pepper and garlic until tender. Stir in the turkey and cook until evenly browned. Pour meat mixture into slow cooker. Min in tomatoes and pumpkin. Season with chili powder, pepper and salt. Cover and set on low, and cook for 4-5 hours. Serve topped with cheddar cheese and sour cream.