Can anyone explain why political red and blue have different meanings in the UK and US?

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In the United Kingdom the colour of the right-of-centre Conservative Party is Blue and that of the left-of-centre Labour Party is red. However, in the United States, although the roughly-centre Democratic party uses the colour blue, the way-off-to-the-right Republican Party fights under the colour red! We all know that the United Stated is a very odd country in many ways, but this seems to be one of the odder aspects. Red is traditionally the colour of left-wing politics - it is the dominant colour of the flags of many Communist countries including China and the former Soviet Union. The British Labour Party sings "The Party's flag is deepest red" at the end of its annual conferences. And yet, in the United States, it is the ultra-right Republican Party, which even thinks that Barack Obama is a Socialist, that uses the colour of Socialism and revolution as its symbol. Why?