I believe there are a number of portraits of Martin Luther by Cranach. Do you mean like this one? That seems fairly typical. It appears he's wearing academic garb, a doctoral cap or academic tam hat or something similar, and a robe similar to a Geneva gown. This is consistent with what I've read, that later in his life Luther mostly gave up clerical/priestly garb like a cassock or a monastic robe, and took to wearing his academic garb (in his capacity as Doctor and Professor of Bible at Wittenberg University). At that it was certainly common that professors wore such robes in daily life, rather than how it today, only for formal occasions like graduation.

So it depends on what age Luther you're going for. If you want later Luther, then academic wear. If you want early Luther, like when he posted the 95 Theses, a monk's robe would be more appropriate. I believe the Augustinian friars, of which Luther was one, wore black, as in that picture.