Red is the color that has the longest wavelengths of any color visible to the human eye. It catches peoples attention and is therefor often used in warning signs. it is also the most used color on many national flags, including the United States of America, Canada, and Russia.
During parts of the Red Scare it was illegal to wave a red flag.
Red in politics
The association of red with the Republicans and Blue with the Democrats only came main-use around 1996 or 1998 during the midterms -- back then, the colors were not standardized, and could refer to any party. For the US map used in many election broadcasts prior to 1996, red was associated with Democrats, while Blue was associated with Republicans.