In a large bowl, combine calamansi juice, vinegar, garlic, ginger, soy sauce, and annatto oil to prepare the marinade.
Add the chicken and coat it evenly. Cover and marinate for at least 4 hours or overnight for the best flavor.
Preheat the grill to medium heat and lightly oil the grates.
Place the marinated chicken on the grill and cook for 10–12 minutes per side.
Baste the chicken occasionally with the remaining marinade or annatto oil while grilling to keep it moist.
Continue grilling until the chicken is fully cooked and has a lightly charred exterior (internal temperature of 75°C/165°F).
Let the chicken rest for a few minutes before serving. Enjoy hot with steamed rice and your favorite dipping sauce.