Whites.

Trumbull, The Death of General Warren at the Battle of Bunker's Hill, 1778

Don't fire until you see the whites of their eyes.

Colonel William Prescott, hollered this day in 1775 during the Battle of Bunker Hill.

Drury,  Plan of Boston and its Environs Shewing the True Situation of His Majesty's Army and Also Those of the Rebels, 1775

The American Battlefield Trust always does an outstanding job of telling the story ...

The Journal of the American Revolution has more.