Pour 1 cup sweet and sour sauce on top of the chicken and seal bag (save the rest of the sauce for basting while grilling).
Put chicken in the fridge and let marinate for at least 3 hours (or up to 24 hours).
If using bamboo skewers, soak them in water for at least 20 minutes before using to prevent burning.
After chicken is finished marinating, thread chicken, peppers, pineapple, and onion onto skewers.
After all the chicken, peppers and onions are threaded on skewers, discard bag with marinade.
Place skewers on grill over medium heat and cook until chicken is completely cooked through and vegetables are tender, turning a couple times to make sure each side is cooked.
Baste as needed with remaining sweet and sour sauce.