The CO LFG (Low Fare Guarantee) applies only if you find the exact same itinerary (flight numbers, day) from another pricing source on the same day that is priced $10 or less. CO will match the price and give you a $100 voucher. CO must be able to find the fare pricing on Expedia, Orbitz, or whatever site is pricing the fare cheaper.

Must be booked in the same fare class.
(You can't specify fare class when selecting your CO flights.)

Your CO ticket/itinerary must be booked/purchased.