Michael Curtiz’ 1930 Al Jolson musical “Mammy” — with restored two-strip Technicolor numbers that are said to comprise about 20 percent of the 84-minute running time — will debut as part of the Warner Archive Collection on April 6. The film’s restoration was undertaken in 2003 by UCLA from a nitrate print found two years earlier at the Netherlands Film Institute in Amsterdam. That print had Dutch title cards, which UCLA replaced with black-and-white footage tinted in sepia. For the DVD release, the sepia footage has been computer-colored to match the original Technicolor. And it’s been combined with a new digital restored master from the original black-and-white negative in the Warner Bros. vaults. This murder mystery set at a minstrel show is the seventh of Jolson ‘s often politically-incorrect musicals available from the Warner Archive Collection.