From Wiki - so take it for what it is:

The advent of color television prompted television news reporters to rely on color-coded electoral maps, though sources conflict as to the conventions they followed. One source claims that in the six elections prior to 2000 every Democrat but one had been coded red. It further claims that from 1976 to 2004, the broadcast networks, in an attempt to avoid favoritism in color-coding, standardized on the convention of alternating every four years between blue and red the color used for the incumbent party.[8][9]

According to another source, in 1976, John Chancellor, the anchorman for NBC Nightly News, asked his network's engineers to construct a large illuminated map of the USA. The map was placed in the network's election-night news studio. If Jimmy Carter, the Democratic candidate that year, won a state, it would light up in red; if Gerald Ford, the Republican, carried a state, it would light up in blue. The feature proved to be so popular that four years later all three major television networks would use colors to designate the states won by the presidential candidates on Election Night, though not all using the same color scheme. NBC continued to use the color scheme employed in 1976 for several years. NBC newsman David Brinkley famously referred to the 1980 election map outcome as showing Ronald Reagan's 44-state landslide as resembling a "suburban swimming pool".[10]

CBS, from 1984 on, used the opposite scheme: blue for Democrats, red for Republicans. ABC used yellow for one major party and blue for the other in 1976. However, in 1980 and 1984, ABC used red for Republicans and blue for Democrats. In 1980, when independent John B. Anderson ran a relatively high-profile campaign as an independent candidate, at least one network provisionally indicated that they would use yellow if he were to win a state. Similarly in 1992 and 1996, at least one network would have used yellow to indicate a state won by Ross Perot.

By 1996, color schemes were relatively mixed, as CNN, CBS, ABC, and The New York Times referred to Democratic states with the color blue and Republican ones as red, while Time and The Washington Post used an opposite scheme.[11][12][13]

In the days following the 2000 election, whose outcome was unclear for some time after election day, major media outlets began conforming to the same color scheme because the electoral map was continually in view, and conformity made for easy and instant viewer comprehension. On Election Night that year, there was no coordinated effort to code Democratic states blue and Republican states red; the association gradually emerged. Partly as a result of this eventual and near-universal color-coding, the terms "red states" and "blue states" entered popular usage in the weeks following the 2000 presidential election. After the results were final, journalists stuck with the color scheme, as The Atlantic's December 2001 cover story by David Brooks entitled, "One Nation, Slightly Divisible", illustrated.[14]

Thus, red and blue became fixed in the media and in many people's minds, despite the fact that no "official" color choices had been made by the parties.[15] As a result, the Blue color attribution has returned to its earliest historical roots in the U.S.: as the Republicans have assumed the legacy of the old Confederacy since their takeover by the Dixiecrats in the 1970s, the Democrats have inherited the Blue of the old Union. However, Archie Tse, The New York Times graphics editor who made the choice when the Times published its first color presidential election map in 2000, provided a different rationale: "Both Republican and red start with the letter R," he said.[16]