Larkspur, eh? Home of the Rennaissance Festival, been a few years since I've been down there.
Sorry, shoulda been a little clearer, the North Platte River Basin. I believe that is the limit of the Iowa darter's native range in Colorado. Anything within two hours of ya will probably limit ya to johnny darters. A little homework should find ya a water body within two hours east of ya that has some. Colorado also has Arkansas and orangethroat darters, in areas of the east, but both are considered threatened within the state and ya would wanna do some serious checkin' with the game and fish before collectin' either.
They are easy to catch. My favorite spot is 'bout 10 miles from the Colorado border in Wyoming, on the Big Laramie. There's a lot of twists in the river at this spot and it creates a lotta pools to choose from. Like I said, as river levels drop, they get trapped in pools along the shore and between sandbars. I look for pools 'bout 18" deep and 3'-6' in size with some rock and plant cover in 'em, then net the darters out with a 4"x6" aquarium net. Forget tryin' to get 'em out of the main river unless ya wanna a work out, they're gone before ya can blink and ya loose sight of 'em almost instantly in the current, not to mention figting the current tryin' to maneuver on slippery rocks. The johnny darters are a little different. They can often be found in small lakes along the shoreline in shallow, gravel and rock strewn waters. A bigger net isn't required, but it sure does make it easier.
The first time I kept darters I kept 'em with standard community fish, though that was twenty-five years ago and I don't remember exactly what. I won't do that again. I've found they're much more active and interesting in an unheated species tank with some zebra danios or white clouds as dithers. WC fish won't take flake or small pellets IME, so I fed a variety of live and frozen foods: blood worms, mysis, etc. If ya can keep 'em somewhere 15°-20° cooler than room temp for a coupla months in the winter, they'll take on breeding colors in the spring. I never could entice either species to actually spawn, though they did display a lot.
Good luck if ya give it a whirl,
WYite