Whay does Saruman use the White Hand as a flag for Isengard?

The hand symbolized one all powerful ruler. It was white because he was the white wizard.

Yeah, that's right...it is the White Hand because Saruman was the White Wizard and Isengard is his place...so the White Hand is Saruman's mark for all Sauron's enemies to know that the Orks and all that beasts are fighting for him...