Very easy: US. 50 from Salida to Grand Junction (gets a little harder once you hit the Utah border, carry water)
Very easy: Rt. 135 from Gunnison to Crested Butte
Easy to Moderate: along 24 and 285 north of Salida
Difficult: 92 and 133 between Carbondale and Delta (Low Traffic)
Easy: 550 Montrose to Durango
Note: When hitchiking in Western Colorado don't get dropped off around Bunea Vista or Canon City, Prisons are located here and you are likely to be stranded for long periods.