Blue is perhaps one of the colors that is most strongly associated with peace. It is the color of tranquility, calm, as well as loyalty, faithfulness, and trustworthiness- concepts all of which are more or less related to a person having peace of mind. In Western culture, there is a specific reason why blue has become a symbol of peace. It is the color of the United Nations flag, created in response to World War II.

Source: Symbols and Meanings