This might help:
http://4wheeldrive.about.com/od/offroadatvbasics/a/where-do-i-register-my-atv-by-state.htm
Out-of-state OHVs
Out-of-state residents who bring OHVs into Colorado must purchase a Colorado Non-Resident OHV permit which is valid from the date of purchase through the following March 31. These permits can be purchased anywhere you get your hunting/fishing licenses in Colorado, via mail-in request, or you can order one online at the Colorado State Parks E-Store.
Once purchased, this sticker should be placed on the vehicle’s outside face of the right front fender or fork. Since this is only a use permit, no renewal notice will be sent to non-resident permit holders; you must purchase a new permit each year you come to Colorado with your OHV.
Decal Placement
Registration decals must be affixed permanently on the upper forward-half of the OHV in a location where the decal can be easily seen. Suggestions: On ATVs the decals could be placed on the outer side of the fenders, on the tank or on the cowling. On bikes, place a decal on each of the outside faces of the upper end of the forks or on the tank. OHV operators must carry their registration card and show it to any peace officer on request. For general decal placement instructions for all types of OHVs and boats, see the Registrations Decal Placement Brochure. View the specific decal placement illustrations for these vehicles:
