Mental Floss: "These days, red means Republican and blue means Democrat, but it wasn't always so. The first color-coded electoral map, which appeared on NBC as part of the 1976 election, showed states that went to Democratic challenger Jimmy Carter red, while states that supported incumbent Gerald Ford were blue. Individual news outlets could decide what colors to assign the candidates--in 1976, ABC's map used yellow for Ford, blue for Carter, and red for states where votes weren't yet tallied--and the colors often flip-flopped from election to election."
"Until the 2000 election between George W. Bush and Al Gore. According to the Washington Post, the first use of "red states" and "blue states" occurred about a week before the election on the Today Show."