In 2 1/2- or 3-quart saucepan heat oil over medium-high heat; add annatto seeds and cook until seeds release their color, 1 to 2 minutes. Remove and discard seeds. To saucepan add onions, bell pepper, and garlic and saute until onions are translucent, 3 to 4 minutes; add water, chicken, tomatoes, broth mix, oregano, and pepper. Squeeze lime halves into soup, then add squeezed halves and grapefruit peel to soup and stir to combine. Reduce heat to low and let simmer for 15 minutes.
Using slotted spoon, remove and discard lime halves and grapefruit peel; let soup simmer for 15 minutes longer. While soup is simmering, spray 9-inch skillet with nonstick cooking spray and heat over medium heat; 1 at a time add tortillas and cook, turning once, until lightly browned on both sides. Cut tortillas into thin strips. To serve, ladle soup into 4 bowls and top each portion with 1/4 of the tortilla strips. Makes 4 servings, about 1 1/2 cups each.