Here are some links to local parks and trails for watching butterflies:

Boulder County Open Space
https://bouldercounty.gov/open-space/parks-and-trails/

Clear Creek County Trails
https://www.co.clear-creek.co.us/884/Trails 

Douglas County Trails
http://www.douglas.co.us/dcoutdoors/trails/open-space-trails/

Gilpin County Trails
http://www.mountaintravelguide.com/Colorado/Gilpin/Hiking/GilpinHiking.htm

Jefferson County Trails
http://jeffco.us/open-space/parks/

Larimer County Parks
https://www.larimer.gov/naturalresources/parks

Park County Trails
http://www.mountaintravelguide.com/Colorado/Park/Hiking/ParkHiking.htm 

Roxborough State Park
https://cpw.state.co.us/state-parks/roxborough-state-park