In a large bowl, combine pineapple juice, soy sauce, brown sugar, garlic, ginger, black pepper, and red pepper flakes. Whisk until well mixed.
Place the chicken thighs in the crockpot and pour the sauce mixture over them, ensuring the chicken is well coated.
Add the diced bell peppers and onions on top of the chicken.
Cover and cook on low for 6-8 hours or on high for 3-4 hours, until the chicken is tender and fully cooked.
Once cooked, shred the chicken in the crockpot using two forks and stir to combine with the sauce and vegetables.
Serve the sweet Hawaiian chicken over cooked rice and garnish with chopped green onions.