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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.

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.