Melt margarine in large saucepan over medium heat. Add onion, celery and garlic; cook and stir 3 to 4 minutes or until vegetables are crisp-tender.
Add white potatoes, sweet potatoes and broth. Bring to a boil. Reduce heat to low; cover and simmer 15 to 20 minutes or until potatoes are tender.
In medium bowl, combine 1/2 cup of the milk and flour; mix until smooth. Stir in remaining milk, salt and ground red pepper until well mixed. Stir into soup; cook over medium heat for 3 to 5 minutes or until bubbly and thickened, stirring constantly.
To serve, ladle soup into 4 individual soup bowls. Sprinkle each with green onions.