Baz Luhrmann’s “La Boheme” opens in a flash of color on Broadway. “The show is far more respectful of its sentimental operatic essence than many of the lugubrious, experimental productions of old war horses at the Met. (Think ‘Lucia di Lammermoor’ or ‘Il Trovatore.’) What Mr. Luhrmann and his extraordinary production designer (and wife), Catherine Martin, have done is find the visual equivalent of the sensual beauty and vigor of the score.”