Why is the United Kingdom white red and blue?

The flag of the United Kingdom is called the Union Jack or the Union Flag. It is made up of the flags of three countries that make up the United Kingdom. The white cross on a blue background is the flag of Scotland, the red cross on a white background is the flag of England and the red saltire on a white background is the flag of Northern Ireland. The Union Jack also contains the arms of Wales, which features a red dragon on a white and green background.