The dirt road up the mountain is easily doable with 2-wheel drive.
We went up Nevado de Colima last month. There is a 9km round-trip trail that is fairly easy to follow, once you find the trailhead near La Joya. The trail goes through pine forest and then ascends over ash / sand to the saddle between the peak and the antennas. Coordinates for the trailhead are: N19° 34.990′ W103° 36.073′
Alternatively, many people seem to drive towards the antennas (currently you're not allowed to drive all the way there), walk the rest of the dirt road to the antennas, and follow a fairly easy path to the top.
More detail on the longer route can be found on our website: