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 brown. Drain, and mix in tomatoes and pumpkin. Season with chili powder, pepper, and salt. Reduce heat to low, cover, and simmer 20 minutes. Serve topped with Cheddar cheese and sour cream.