In a small bowl, combine yogurt, cilantro, lemon juice, lime juice and 1/8 teaspoon salt; set aside. In a large skillet, heat oil over medium-high heat. Add peppers, chiles and onion; cook and stir until crisp-tender, 4-5 minutes. Add shrimp, garlic, cumin and remaining ¼ teaspoon salt. Cook and stir until shrimp turn pink, 2-3 minutes. Remove from heat; serve in tortillas with lettuce, yogurt sauce and, if desired, lime wedges.