In terms of bang for the buck, you might want to consider the 20 flies for 9.99 in the Orvis banner ad that pops up on this site-- it's a decent all around assortment for trout every where-- and many of the patterns are useful in CO.
But you'd also do well to add a few locally tied flies that are recommended by gurus from area fly shops like Charlie Craven from Charlies Fly Box in Arvada, or Pat Dorsey from Blue Quill Angler for the South Platte, Clear Creek and other area waters-- some of that water can be quite technical and require some pretty small flies and or fishing with some dry/dropper combinations.
For example here's a link to Pat Dorsey's recommendations for summer hatches and fly patterns:
Colorado Stream Reports for Fly Fisherman at the Blue Quill Angler
So for example in addition to the assortment of all around trout patterns in the Orvis 20 flies for $9.95 deal, you might want to add some other specific stuff from time to time ( as well as replacements for the stuf you leave in trees behind you) when you can like:
PMD Thorax 16 for Pale Morning Dun Mayflies
Crystal Stimulator 10 (a good imiation for stoneflies and a good top fly for dropper rigs)
some small dries for midges and blue wing olives
Snowshoe Suspender Midge 22 (dry fly)
BWO Sparkle Dun 20 (matches Baetis and other Blue Wing Olives mayflies)
flies for nymphing and the bottom fly on dropper rigs
Barr's Graphic Caddis 18 (emerging caddis)
Mercury Midge 20 (subsurface midge larva/pupa) you could fish off as a dropper)
Mercury Flashback Pheasant Tail Nymph (Baetis and other Blue Wing Olive nymphs)