Combine pesto, lemon juice, and shrimp; toss. Let stand 5 minutes. Thread shrimp and red and yellow bell pepper alternately onto skewers. Sprinkle evenly with salt. Place skewers on a grill rack coated with cooking spray. Grill 7 minutes, turning skewers occasionally for an even char.
Note: If using wooden skewers, soak them in water for 20 minutes before grilling.