David Robbins's Endworld series. Specifically the green spots are in Dallas Run, which is the 20th book in the series.
The greenspots are a mark of the devotees of cult, and they use an old stadium as their temple, which is why you remember the stadium. The protagonist Blade first discovers the existence of the cult when he meets a cult member Marta:
"We can't take any chances," Sergeant Whitney said. "There's a possibility you're infected with a disease."
"I am not," Marta responded indignantly.
"Then what are those green spots all over you?"
Marta touched one of the inch-wide irregular splotches on her left thigh. "Do you mean these?"
"What are they?"
She traced the outline of the splotch with her finger. "Isn't it beautiful?"
"Miss?"
"Such exquisite design. The Maker is magnificent."
The head of the cult is called The Lawgiver:
Blade couldn't stop himself from doing a double take. Like the rest of
the Chosen, the Lawgiver's body was covered with the green splotches.
Unlike the others, the elderly man's face was a shiny shade of green from his forehead to his chin. And what a face! The visage resembled a
predatory bird of prey, an eagle or a hawk. A great, hooked nose divided a perpetually puckered pair of thin, cruel lips and a pair of eerie, dazzling green eyes. Wrinkles creased the Lawgiver's forehead and cheeks, suggesting an age well beyond the normal life expectancy.
