legal question on a seat belt ticket i recieved

[cap'n cook]

not sure if this should go here but, it didnt seem like it should go in legislation and politics?

today i got pulled over for going 15mph over in a 45 mph long straight road with fields on both sides. the car is an older toyota camry with the automatic shoulder belt which i was wearing. the officer let me off of the speeding part, which i am thankful for, but he gave me a $75 dollar ticket for not having the lap belt on (automatic shoulder belt on, lap belt not on). does anyone know if the lap portion of the belt is required by law here in Co. i know that if driving a vehicle equipped with seat belts then the front seat driver and passenger are required to wear them, but the officer said if the vehicle is equipped with seat belts you are required to wear ALL the belts, meaning both lap and shoulder belts. is he correct on this or can i fight it? i really dont have an extra $75.

thanks

edit. i did look up the seat belt laws, but cant find an answer on this.