In a large mixing bowl, combine orange juice, lime juice, olive oil, salt, garlic powder, oregano, sugar (if using), onion powder, cumin, and black pepper. Whisk until well blended.

Place chicken pieces in a zip-top bag, pour marinade over, seal tightly, and refrigerate for at least 2 hours or overnight.

Soak bamboo skewers in water for one hour.

Chop zucchini, yellow squash, and onion into uniform pieces. Toss with olive oil, salt, garlic powder, oregano, and black pepper.

On soaked skewers, alternate marinated chicken and seasoned vegetables without compressing them.

Preheat grill to medium (350°F to 375°F).

Grill the skewers for 8 to 10 minutes, turning occasionally until chicken reaches 165°F.

Let skewers rest for a moment before serving hot.