Continuity mistake: Nicholas Hammond (Friedrich) was not a natural blond. The first scene to be shot involving him was the "My Favourite Things" scene and you can see that his hair is pale auburn. After this scene was shot, the directors thought that he would look better if his hair were lighter, so they applied an early form of bleaching agent to his hair. In scenes which were shot after this (notably the children's first entrance to meet Maria which occurs before the "Favourite Things" scene) his hair is very, VERY blond, almost white. In fact, if you scrutinise his hair closely throughout the film, it is a different colour in practically every scene, depending on when the scene was shot and how much of the bleaching agent had been applied.