Commentary Purg XIII 43-48

The coloration of the garments worn by the penitents in Envy is so close to that of the rocks themselves that it has taken Virgil a long while to make them out; he only now calls them to Dante's attention -- and the protagonist has had no sense of their presence at all.

      Envy, which is so clearly aware of difference, is cloaked in a uniform that makes distinction of difference nearly impossible.  As Dante interacts with these penitents, it is clear that he has a hard time making out the features of any one of them.