Chuck
I don't know about Colorado, but in Utah, I see the side by side Utes on the Jeep trails, and the streets in Moab, but they have license plates. The regular ATVs are more limited I believe.
Here is the official word

http://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1520&context=govdocs

Peter
_________________________
Peter Kaufman
NRA Life
GOA
SAF
RKCC CM-023
RKS #5642
SHAG # 005
Ironwood Nut
Life is too short to have ugly knives