I love it when photographers are properly open with technique stuff like this, and don't give it all the coca- cola secret recipe tale; although truth be told i struggle to be open about process:
'AFH: What about this next photograph [Untitled, Morrissey 23]?
RM: Our nickname for this one is "the hulk" because he's all green.
AFH: How are you getting these colors?
RM: The colors are the result of film experimentation. Before I go to the concerts, I expose the film to different lights - Daylight, television light, tungsten light, sunsets, colored lights, morning light, basically all kinds of light. I expose the roll and then shoot it again at the shows. Between exposing the film and utilizing the intense stage lighting, i've got a process down to achieve new color palates.'
(from an interview with ryan mcginley on the saatchi blog)