Red
They are the colours of the olympic rings. The five colours together with the white of the background represent the colours of the countries participating when the present Games were established - "the blue and yellow of Sweden, the blue and white of Greece, the tricolour flags of France, United Kingdom, the United States, Germany, Belgium, Italy and Hungary, and the yellow and red of Spain are included, as are the innovative flags of Brazil and Australia, and those of ancient Japan and modern China".
