Final answer:
The color on the bottom of the center symbol of the horizontally hung Korean flag, the Taegukgi, is red.
Step-by-step explanation:
When the Korean flag is hung horizontally, the color on the bottom of the center symbol is red. The Korean flag, known as the Taegukgi, features a circle in the center with two parts: the upper (red) part represents the proactive cosmic forces of the yang, and the lower (blue) part represents the responsive cosmic forces of the yin. It is important to hang the flag correctly to respect the symbolism and the cultural significance it holds for the Korean people.