Citations in the World Almanac (listed by cover date; color information is from the previous year): red/yellow (1895-1897); red/gold (1900-1904); crimson/gold (1906-1918); red/gold (1923-1931); crimson/gold (1934-1935)
According to the University of Denver Bulletin (May 1914), students were required to wear academic costume to commencement ceremonies, so it is likely that the Intercollegiate Bureau of Academic Costume (IBAC) had assigned the university a hood lining by that point at the latest. The first IBAC description of Denver’s hood lining is from 1927, where it was said to be cardinal (not crimson) with a gold chevron. By 1969 the IBAC had corrected the shade of red to crimson.
In 1885 the official colors of the University of Denver became “alizarin crimson” and “lemon yellow”, sometimes described more simply as scarlet and maize. But in the 1890s the yellow began to be replaced by gold in popular usage, so in 1947 a faculty committee officially changed the colors of the university to crimson and gold. Crimson was chosen because Colorado means “red earth” in Spanish, and gold because it is a precious metal that had been panned from the mountains of the state.