I think it is like this, correct me if I'm wrong. The Deluxe seat belts were a separate option from the Deluxe interior.
So, a car with standard interior could be had with either the standard seat belts or ordered with Deluxe seat belts. Likewise, a car with the Deluxe interior could be had with the standard seat belts or ordered with the Deluxe seat belts.
The standard seat belts were made in fewer colors than the Deluxe seat belts, see the link to the Camaro site above ( http://www.camaros.org/seatbelt.shtml#color ). No red or turquoise standard belts are mentioned. So a car with a red interior, no matter if it was the standard interior or the Deluxe interior, would get black belts if the standard belts were fitted. If the same car was ordered with the optional Deluxe belts, it would get red belts.
