Fill Dutch oven about half full of water. Cover and heat over high heat until boiling. Add pasta shells. Heat to boiling again. Boil uncover 11-13 min. While boiling, continue with recipe.
Peel and chop the onion. Cook the beef and onion in skillet over med-high heat 5-6 min. stirring occasionally, until beef is brown; drain. Stir chili powder, diced tomatoes, cream cheese and 1/4 cup taco sauce into beef mixture. Heat over med-low heat 2-3 min. stirring occasionally, until cheese is melted. Remove from heat.
Heat the oven 350. Spray pan with cooking spray. Drain pasta shells. Fill the shells with the beef mixture, using about 2 Tbsp. for each shell. Place in sprayed pan. Pour 1/2 cup taco sauce over the shells. Cover with aluminum foil and bake 20 min. Remove pan. Sprinkle the Colby-Monterey Jack cheese and corn chips over shells. Bake uncovered about 10 min. until cheese is melted. Peel and slice the green onion. Garnish pasta shells with sour cream and green onions.