Royal blue describes both a bright shade and a dark shade of azure blue. It is said to have been invented by millers in Rode, Somerset, a consortium of which won a competition to make a dress for the British queen, Charlotte of Mecklenburg-Strelitz. Traditionally, dictionaries define royal blue as a deep to dark blue, often with a purple or faint reddish tinge.

By the 1950s, many people began to think of royal blue as a brighter color, and it is this brighter color that was chosen as the web color "royal blue" (the web colors when they were formulated in 1987 were originally known as the X11 colors, since the World Wide Web did not come into operation until 1991). The World Wide Web Consortium designated the keyword "royalblue" to be this much brighter color, rather than the traditional darker version of royal blue.

When, in 2008, the United States Transportation Security Administration changed the color of airport screener uniforms from white to royal blue, they found that it made the work of the airport screeners easier because airline passengers became more compliant, apparently because by wearing blue, the airport screeners came to be perceived more as authority figures.