Red
The red flag appeared as a political symbol during the French Revolution, after the fall of Bastille. A law adopted by the new government on October 20, 1789 authorized the Garde Nationale to raise the red flag in the event of a riot, to signal that the Garde would imminently intervene. During a demonstration on the Champs de Mars on July 17, 1791, the Garde Nationale fired on the crowd, killed up to fifty people. The government was denounced by the more radical revolutionaries. In the words of his famous hymn, the Marseillaise, Rouget de Lisle wrote: "Against us they have raised the bloody flag of tyranny!" (Contre nous de la tyrannie, l'entendard sanglant est leve). Beginning in 1790, the most radical revolutionaries adopted the red flag themselves, to symbolize the blood of those killed in the demonstrations, and to call for the repression of those they considered counter-revolutionary.

Q: On what date was the Garde Nationale empowered to hoist a red flag to signal their intervention in a riot?
October 20, 1789