Place chicken breasts or thighs in the slow cooker.
In a bowl, mix together teriyaki sauce, brown sugar, and the reserved 1/4 cup pineapple juice.
Pour the sauce mixture over the chicken, then add pineapple chunks and bell pepper pieces on top.
Cover and cook on low for 5-6 hours or high for 2.5-3 hours until chicken is cooked through and tender.
Shred or slice the chicken and serve over white rice or cauliflower rice with extra sauce spooned on top. Garnish with green onions if you're feeling fancy!