On many of CO's Boeings, it's on the side of the armrest, in the back, covered by the seat back cushion. If you feel between the cushion and the armrest, you'll probably find it.

If you look at the aircraft accessibility page on the CO website, you can actually find directions for locating the armrest release on the different types of seats CO uses, along with descriptions of which seats are on which equipment. The individual seat maps also have information on which rows have non-movable aisle armrests.