Japan has one of the world's most distinctive flags, a red circle on a white background. This is intended to represent the sun, and the flag is called the Hinomaru in Japanese: the circle of the sun.
After World War Two there were attempts by the allied powers to force Japan to change its flag. It was claimed that the Hinomaru was a symbol of Japanese nationalism. That was obvious nonsense, so the Japanese refused to change their flag. It's been used since the 7th Century, alongside other flags that represented the family crests of ruling families. In 1854 it was recognised as the only valid Japanese flag.
The average Japanese citizen feels less connection to his flag than the citizens of other countries. The Hinomaru is used at official ceremonies, but rarely at home or on public buildings. The Japanese government wants to promote the use of the flag as a symbol of national pride, but its use is only recommended, not enforced. This is in contrast to the policy before World War Two, when all private homes were required by law to display the Hinomaru on national holidays.
There have been many arguments about the exact colour of red circle. Originally it was specified merely as "dark red", usually the darkest red dye available to printers. In our day of digital computer technology the colour had to be finally defined. In 2008, after long discussions, the RGB colour #ed1b2f was chosen as the recommended colour. It's not the darkest red, but it's supposed to be the most sun-like shade of red.