Colorado's flag is close to a monogram: three stripes of blue and white with a big red C at the hoist, a golden ball set inside the letter's curve. It is one of the few US state flags built frankly around a letter of the alphabet, which flag design orthodoxy frowns on, and one of the few that people actually put on their snowboards and beer cans anyway. The design dates to 1911 and has survived unchanged in substance, though it took the legislature two more laws and 53 years to say exactly how big the C should be.
Carson's design and the banner it replaced
Colorado's first attempt at a flag was the standard failure mode of American state flags. In 1907 the state adopted a banner, promoted by the Denver chapter of the Daughters of the American Revolution, consisting of the state coat of arms on a dark blue field: dignified, indistinguishable at thirty paces from twenty other state flags, and unloved.
Andrew Carlisle Carson's alternative arrived four years later and won. The General Assembly adopted his design on June 5, 1911. Carson dropped heraldry entirely in favor of geometry: equal stripes of blue, white, and blue, a red C reaching from the top stripe to the bottom one, and a disc of gold filling the letter. Nothing on the flag requires explanation to be read as Colorado, but everything on it has one.
Sky, snow, earth, and sun
Carson's color scheme is unusually literal. The blue stripes are the Colorado sky. The white stripe is the snow on the Rocky Mountains. The red of the C is the state's reddish earth and rock, which is the whole reason the state is called Colorado: Spanish explorers named the Colorado River for the red silt it carried, and "colorado" means colored red. The golden disc is the sunshine the state advertises at every opportunity.
There are two second-order readings built in. Gold and white also refer to the gold and silver mining that created Colorado in the first place, from the 1859 Pikes Peak gold rush through the silver boom towns. And blue and white are the colors of the Rocky Mountain columbine, the state flower since 1899. For a flag with only four colors and one letter, it covers a lot of physical geography, and it does so without a single word or seal.
Two fixes: 1929 and 1964
The 1911 act described the flag in prose and left the numbers out, and for decades Colorado flags disagreed with each other. What shade of blue? How big is the C? Where exactly does it sit? Manufacturers answered differently, and photographs from the era show Cs ranging from modest to enormous, positioned anywhere from the hoist to dead center.
The General Assembly fixed the colors first. An act of February 28, 1929 declared that the flag's blue and red are the same as the blue and red of the United States flag, ending the shade lottery. The geometry waited another 35 years, until an act of March 31, 1964 dimensioned the design: the flag's proportions settled at 2:3, and the diameter of the golden disc was set equal to the width of the white center stripe, which also governs the size of the C around it. Since 1964 a Colorado flag from any maker should look like a Colorado flag from any other, which sounds like a low bar until you learn it took 53 years to clear.
A state flag people choose to wear
In the North American Vexillological Association's 2001 survey of 72 state and provincial flags, Colorado ranked 16th with a score of 6.83 out of 10, in the upper quarter despite the letterform at its center. NAVA's design principles warn against lettering on flags, but a single bold initial works differently from a printed state name: the C functions as a shape first and a letter second, closer to a logo mark than a caption.
The market agrees. Colorado's flag has escaped government flagpoles more completely than almost any other state flag. The C-and-disc appears on ski passes, brewery taps, gym shirts, trucker hats, and the state's own license plate options, and a stylized version brands everything from tourism campaigns to sports merchandise. Along with New Mexico's zia sun and Texas's lone star, it belongs to the short list of state flags that double as commercial identities. Not bad for a design whose main legislative drama was a 53-year argument about the diameter of a circle.
References
[1] "Flag of Colorado," Wikipedia. https://en.wikipedia.org/wiki/Flag_of_Colorado
[2] "Colorado," US Flags dot Design. https://usflags.design/colorado/
[3] Colorado Revised Statutes, section 24-80-904, state flag.
[4] "History of the Colorado State Flag: Origins, Meaning, and Evolution," Baldwin Flags. https://www.baldwinflags.com/blog/american-state-flags-11/history-of-the-colorado-state-flag-origins-meaning-and-evolution-143
[5] Ted Kaye, "The American State/Provincial Flag Survey," Raven: A Journal of Vexillology, Vol. 8 (2001), NAVA.