Combine the chicken with the paprika, sazon, soy sauce, cornstarch, and black pepper; let marinate for at least 10 minutes.
Heat the vegetable oil in a large pan or wok over high heat and stir-fry the chicken until cooked through; remove and set aside.
In the same pan, stir-fry the bell pepper, onion, and garlic until crisp-tender.
Whisk together the soy sauce, oyster sauce, dark soy sauce (if using), sugar, and water; add to the pan along with the cooked chicken.
Toss everything together until coated and the sauce thickens slightly.
Garnish with chopped green onions and serve.