Symbolism, Imagery, Allegory
In a story as short as "The Revolutionist," anything that gets mentioned more than once is trying to tell us something. And since Mantegna comes up three times—twice in reference to the comrades distaste for the painter and once in reference to the narrator's fondness for his work—dude's definitely doing some symbolic work. Want to know what, exactly, this is? Hop on over to the "Characters" section and read up about the comrade; we've got plenty to say about Mantegna over there.
Though Hemingway doesn't specify which Mantegna paintings the narrator admires and the young man abhors, here is one painting that's housed in Milan, which is the place the comrade is headed when the narrator recommends he check out Mantegna's work. Though the comrade has already made up his mind about Mantegna—which means he's seen the painter's pieces someplace beside Milan—it might be interesting to consider why the narrator would like this painting, as well as whether there's anything in it that seems like it would be off-putting to the comrade.