https://inews.co.uk/us-election-2020/republicans-red-democrats-blue-what-colour-us-election-results-map-explained-747786:

According to Professor David Scott Kastan of Yale University, writing in The Conversation, the system’s origins lie in the spread of colour TV in the late 1960s, when colour-coded maps were first used on election TV broadcasts.

The red and blue colouring was a nod to the British system, The Verge reports, but initially there was no permanent colour association for either party.

TV networks changed the map coding from election to election, with Prof Kastan explaining: “In Cold War America, networks couldn’t consistently identify one party as “red” – the color of communists and, in particular, the Soviet Union – without being accused of bias.”

Though red and blue have often been used to stand in for opposing sides in U.S. political history, it’s only since the 2000 election that red and blue have been assigned to the political parties consistently.

That year, The New York Times and USA Today published full-color electoral maps for the first time, and according to The Verge, they assigned the colors fairly arbitrarily.

“[R]ed begins with r, Republican begins with r,” senior graphics editor Archie Tse told The Verge. “It was a more natural association.”