French flag: Blue-White-Red
the French Revolution started in 1789, but France did not become immediately a republic, first was established a constitutional monarchy. Before that, there was no official flag but symbols such as the white flag (because white is the colour of monarchy) and fleur-de-lys. When absolute monarchy was abolished, revolutionnaries presented to king Louis XVI (who was still King, but had lost many powers) the tricolour saying that Blue and Red were Paris' colours and that white represented monarchy. Actually the colours were inspired by the American flag.
This flag was officially 'imposed' in 1794 and the article 2 of the French constitution defines it as the national emblem.
@tomekcrr: you are right, white is supposed to symbolize royalty.
Nowadays, as France is a democracy (yes it is ) people prefer saying that the colours refer to the 3 concepts of France's national motto : Liberté, Egalité, Fraternité (Freedom, Equality, Fraternity).