Context does help this a little bit. The paragraph is from "The Plague of Bergamo" from "Mogens and Other Stories" by Jens Peter Jacobsen, as benjamin kar linked to.
"Red dissipated" describes the people of Old Bergamo after being ravaged so completely by plague that they abandon their God and morality. This description contrasts the appearance of a mob of proselytizers who come to Old Bergamo and appear like walking skeletons:
They look as if they came from a famished city, their cheeks are
hollow, their bones stand out, their lips are bloodless, and they have
dark rings beneath their eyes.
The plagued people of Old Bergamo are described in terms of things that are red. Physically from the effects of the weather and plague:
And day by day the plague increased, the summer's sun blazed down upon
the town, not a drop of rain fell, not the faintest breeze stirred.
And morally "so enormous their depravity." Where white is akin to purity, red would be its opposite.
The Old Bergamo people are described as dissipated because they are both physically and morally wasting away.