The National Film Preservation Act, designed to inform consumers when they are about to rent, watch or buy a film that has been altered from its original color or shape, was signed into law Tuesday by President Reagan. The act, which was attached to a huge Interior Department appropriations bill signed by Reagan, was enacted in spite of protests by Motion Picture Assn. of America President Jack Valenti, who in July publicly attacked the bill but "was not lobbying against it." As written by its sponsors, Reps. Robert Mrazek (D-NY) and Sidney Yates (D-Ill.), the act will establish a National Film Preservation Board and empower the board to designate up to 25 films per year as "classics." It will require those who color, re-tint, edit or otherwise change the film from its original state to label the new product as "altered." The Directors Guild of America, through a spokesman, announced it was "quite pleased that, for the first time in history, movies have been recognized as works of art." Valenti was out of the country and unavailable for comment Tuesday.