Fort Collins:  ALL fireworks, including sparklers and snakes, are illegal to sell, possess, or use. (Up to $2750 fine.)  Cross the legal lines of city limits, however, and you will see some fireworks stands, my friend.  And last year was the first year that I enjoyed some from Colorado in ages, and gotta say - the fountains and things were actually pretty fun.  I was surprised!!

Loveland: “Permissible (legal) fireworks include items such as sparklers, fountains, smoke balls, items with crackle and strobe effects, wheels and spinners, and various novelty items. These types of fireworks are allowed to be sold (with permit only) and used within the Loveland city limits.  Fireworks are not to be possessed or used by children under the age of 16 without direct adult supervision.”

Larimer County:  Generally speaking, if they leave the ground or make a sound, they're illegal.

Greeley:  Pretty much the same law as Loveland.

Then, there's Wyoming, where fireworks vendors, of which there are four right off I-25 right after you cross the border going north, say 95% of their business comes from Colorado.

As someone who used to commute daily to Cheyenne, I can tell you that, if you want to buy fireworks from Wyoming, it's a good idea to do so well before July 3rd or 4th.  The cops will be there and they will search!  And even if they don't, you might be sitting still on the interstate for hours.