That’s a relatively recent phenomenon. It goes back to the 2000 election, so it just makes the cut for a question here.

Red state and blue state refer to the electoral college maps used by networks for their election night broadcasts. And despite no coordination, they almost all used the Dem blue, GOP red color scheme. Since that election was so close and contested for weeks after the election, red state and blue state became shorthand for the two.

It is worth noting that the red/blue scheme was not uniform until that election. Many times it was reversed. And during the Cold War, many electoral maps used yellow and blue to avoid comparisons to Communists.

A brief rundown on what each network did before:

NBC used blue for the incumbent party, red for the challenging party.

CBS since 1984 used the Dem blue, GOP red scheme we all know now.

ABC used yellow for GOP, then switched to red in the 1980s. Dems were blue.

NYT mostly used the Dem blue, GOP red scheme.

Washington Post reversed it: Dem red, GOP blue before 2000.