Co. requires a .24 caliber. And you cannot hunt varmints during a regular big game season with anything larger than a .22 center fire if you do not have a current unfilled tag for that big game season. Guess they figured out the party hunters. Once the regular big game season is over the larger calibers are legal for varmints & fur bearers.
Couple of guys I know got a nice fine for not using a smaller caliber to shoot prairie dogs during pronghorn season a few years back.