In the 1970s, 1980s, and 1990s, the red states voted for the Democratic Party and the blue states voted for the Republican Party.
Then the news media, which supported the Democratic Party, decided that Democrats were somehow losing votes, because they were said to be red.
So the colors were changed, to what they now.
Red is a more accurate symbol for the Democratic Party, because their beliefs resemble the former Soviet Union (which had a red flag) and Communist China (which still has a red flag).
Therefore, the original colors were more honest.