Its obviously a lepidoptera but I'm not sure what family this butterfly could be.
This is a "Painted Lady" butterfly, which belongs to the family Nymphalidae, and the species Vanessa cardui.

The following is a distribution map of the butterfly within the US, and then Colorado specifically.


An interactive version of this map can be found here.
Following its annual spring migration the painted lady may be found anywhere in the state. However, it is primarily a species of fields and open areas. They are also common visitors to flowers in yards and gardens. The painted lady is one species that many school children have encountered as rearing one of these is now almost a rite of passage in elementary school classes. It is also the butterfly species commonly used for release at weddings and other celebratory events. source
This in mind, the painted lady is in no risk of being endangered.