The US Republican Party's official color is Red yet in general political circles that color has been used by Socialist groups and movements. Since the party publicly condemns Socialism why would they brand with Red?
As Kevin Drum found (from the Washington Monthly), from 1976 to 2004, the incumbent party's coloring alternated. As it happened, from 1976 through 1996, this meant that the Republicans were the blue party five of six times (1988 was the exception). But no one particularly noticed.
In 2000 and 2004, the Republicans were the red party by that system. And in 2000, it mattered (Kevin Drum from another Washington Monthly article). Because in 2000, the race was close enough that the contested election in Florida mattered. So they spent weeks showing the states broken down under the 2000 color scheme. And because the challenger in 2000 (the Republicans) were the incumbents in 2004, they had the same color scheme even though it alternated. Then 2008 arrived and everyone was accustomed to Republicans being red, so it stuck even though it was the Democrats' turn.
It might not have stuck if it were not that red communism collapsed in the early 1990s. So thinking about red being a communist color was not particularly topical in any of 2000, 2004, or 2008. Bernie Sanders started a revival of socialism in 2016, which continued in 2018. In that context, it seems more important now (in 2019) than it did in 2008 or 2012.
TL;DR: Republicans did not choose red; it was chosen by a historical accident.
4I wouldn't say that the Red = Communism was not topical in the 90s. The USSR collapsed in 1992, just 8 years before the party colors solidified. Phrases like "Commie Pinko" or "Better Dead than Red" were still said often, and I do recall Obama (the only Post Soviet Union Democrat to be elected President... Clinton being elected a month before the USSR fell had to struggle with cold war elections) was called a Pinko and a Red socialist more than a few times.+– hszmvMar 11, 2019 at 12:31
8+ I imagine that the color wasn't associated with Communism when used in this light because neither party was remotely socialist prior to 2000, and the two colors are in the Flag, so have a U.S. Association... White is also seen as the balancing color or neutral color in political color schemes, which is why it wasn't used. Just to verify this source, I do recall seeing a clip of news coverage of Election 1984 as the ballots came in, showing states called for Reagan (Incumbent) in Blue. In a similar story, the Elephant and Donkey were not chosen, but insults that were appropriated.– hszmvMar 11, 2019 at 12:35
8It's particularly weird now hearing pundits talk about states historically being "reliably red" or "reliably blue" since those color associations are not historical. Mar 11, 2019 at 15:00
2What do you mean by "the incumbent party's coloring alternated?" This answer seems to be implying that, before 2000, the incumbent party's color was always blue, but that's the opposite of it alternating. Mar 11, 2019 at 17:51
4A missing bit of background is the choice of red and blue for the parties in the first place allows you to make an entire map (including outlines and numerical figures) using red, white, and blue.– hobbsMar 11, 2019 at 23:23
The usage of the color red for the Republican Party in the US is surprisingly recent. The current color scheme of red and blue didn't become common before the Presidential Elections of 2000. There is a good Washington Post article Red vs. Blue: A history of how we use political colors.
For example, you can see that yellow was used by TV channel ABC for the Republican Party in 1976 (Ford against Carter).
Edit: About the political symbolism of the color red
Today the color red is associated with Communism. This was even more so during the Cold War.
The Soviet Union, (People's Republic of) China, Vietnam and other countries had or still have national flags, which are dominated by red. During the Cold War red was the color of the enemy. Western maps depicted NATO countries in blue and members of the Warsaw Pact in red. Nobody would want red as their color, if they didn't have a good reason (i.e., Socialist and Social Democrat parties - which had already used this color before the Bolshevist Revolution in 1917).
After the disintegration of the Soviet Union and the end of the Cold War it became okay to be associated with the color red. Blue was claimed by the Democrats. Plenty other colors (green, brown, white, purple, ...) have also undesired connotations. Yellow is visually not so attrative - especially in front of a white paper background. So, why not red?
By the way, red is not only the color of Communists. It had been the color of the "flag of defiance". Cities under siege displayed a red flag to show their determination to resist. During the French Revolution the first banner of the revolutionaries was not blue-white-red - but red. The modern French flag became popular as a symbol of reconciliation after French king Louis XVI acceded to the revolutionaries' demands. It merged the red and blue of the city of Paris with the white of the monarch.
5People are focusing on red alone, but that's incorrectly ignoring the blue. The national colors of the US (and several other countries) are red, white, and blue. When red and blue are used together, they take a patriotic, national meaning. Nobody looks at the big red stripes on the US flag and associates that with Communism. Mar 12, 2019 at 6:05
@user71659 Red on National flags are the exception as it is common in that situation means, As Geogrphy now says "The of those that fight for thier freedom" Mar 15, 2019 at 6:11