The state flag of Delaware was adopted in 1913. The colonial blue and buff represent the colors of General George Washington's uniform. "December 7, 1787" is the day Delaware ratified the federal Constitution - becoming the first state of the union.

Inside the buff diamond is the state coat of arms (first adopted in 1777), which contains many symbols of Delaware:

Ship - a symbol of Delaware's ship building industry and extensive coastal commerce.

Farmer - represents the central role of farming to the state of Delaware.

Militiaman - recognizes the crucial role of the citizen-soldier to the maintenance of American liberties.

Wheat Sheaf - a symbol of the agricultural vitality of Delaware.

Maize (Indian Corn) - symbolizes the agricultural basis of Delaware's economy.

Water - represents the Delaware River, the state's main artery of commerce and transportation.

Ox - signifies the importance of animal husbandry to Delaware's state economy.

Motto - "Liberty and Independence" was approved in 1847.

Source:

Delaware Facts & Symbols - State Flag: DE official website

Links:

Delaware State Seal: official website of the 1st State
State Flags - 50 states List
National Flag