Washington's flag is the only state flag on a field of green (an appropriate color for "The Evergreen State"). It is also the only state flag displaying an image of an actual person

The official flag of the state of Washington shall be of dark green silk or bunting and shall bear in its center a reproduction of the seal of the state of Washington embroidered, printed, painted or stamped thereon. The edges of the flag may, or may not, be fringed. If a fringe is used the same shall be of gold or yellow color of the same shade as the seal. The dimensions of the flag may vary.

Note: Washingon is the only state that is named in honor of a United States president.

Source:

Washington (U.S.):Flags of the World
Washington State Flag: Netstate.com

Links:

State Flags - 50 states List
National Flag