The 5-O flag is composed of the colours blue and red. Since blue is the 5th colour rainbow, the blue area in the beginning represents 5. Since 'O' is the '15th letter in the alphabet it is composed of the first and fifth colours of the rainbow which are red and blue. In our case the fly diagonal side represents the flag coloured letter 'O'. Coincidentally US police units flash red and yellow lights on their vehicles when they are in pursuit.