A black flag is the emblem of piracy or of no quarter. (See Black Flags.) To unfurl the black flag. To declare war. The curtain which used to hang before the door of Ayeshah, Mahomet's favourite wife, was taken for a national flag, and is regarded by Mussulmans as the most precious of relics. It is black, and is never unfolded except as a declaration of war.
A red flag. To display a red flag is to defy or dare to battle. Red is the emblem of blood. The Roman signal for battle.
A yellow flag signals contagious disease on board ship. To get one's flag. To become an admiral. Formerly the captain of a flagship was called a “flag-officer.”
“I do not believe that the bullet is cast that is to deprive you of life, Jack, you'll get your flag, as I hope to get mine” —Kingston: The Three Admirals, xiii.
To hang the flag half-mast high is in token of mourning or distress. To hang out the white flag. To sue for quarter, to give in.
To lower one's flag, to eat humble pie to eat the leek, to confess oneself in the wrong; to eat one's own words.
“The ... Association ... after systematically opposing the views of the ... National Congress, had to lower the flag and pass a resolution in favour of simultaneous examinations.” —Nineteenth Century (April, 1894, page 670).
To strike the flag. To lower it or pull it down upon the cap, in token of respect or submission. In naval warfare it means to surrender.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894