You will need to go to the Camionera Nueva en Tonala, the interstate bus station for Guadalajara metropolitan area. There are several modules; small bus stations within the complex, so it is best to know which bus line you will use to go to Colima, in order for the taxi to drop you there. The modules are arranged in a circle around a large parking lot. Expect the taxi to cost a few hundred pesos I would guess.
ETN and Primera Plus and you can check Omnibus go from Guadalajara to Colima. Google etn.com.mx for schedules (the site is not working well right now but it will come back up then check primeraplus.com.mx and omnibus.com.mx look at the schedules but buy your ticket at the station for better flexibility. Get off at the station and check the various bus companies, you can walk from one counter to another and you can buy a ticket with the bus that goes out the soonest. The bus station is about 30 minutes north of the airport in Tonala.
You can buy your ticket at the taxi booth at the airport so you will not be cheated, I would thing it is around 300 pesos but do not remember. The trip to Colima will be reasonnable even on a luxury bus like ETN . or Primera plus, actually the site will tell you. Primera plus is cheaper than ETN and Omnibus is cheaper as well.
Have a nice trip