For the soup: In a medium nonstick skillet, heat oil over medium heat; add onion and salt. Cook, stirring occasionally, until onion is softened, about 5 minutes.
Add broth, bring to a boil, and simmer for 3 minutes; transfer to a medium bowl to cool.
Place half of the cooled broth mixture in a blender (make sure to get an equal amount of liquid and onions); add yellow peppers and 1⁄2 cup of the yogurt; purée until smooth.
Transfer yellow pepper purée to a covered container and refrigerate until chilled, about 30 minutes.
Rinse blender and transfer remaining broth mixture to it. Add red peppers and remaining 1⁄2 cup yogurt; purée until smooth.
Transfer red pepper purée to another covered container and refrigerate until chilled, about 30 minutes.
For the salsa: While soups are chilling, in a small bowl combine avocado, cilantro, and lime juice.
To serve the soup, simultaneously ladle a generous 1⁄2 cup of the yellow pepper soup and a generous 1⁄2 cup of the red pepper soup into each of 4 shallow bowls, allowing soups to meet in center. Garnish each with salsa and serve.