When the paint is applied to a person's photograph - adding gray to the hair or wrinkles to the face - the same thing happens to the person physically, causing them to age - potentially to the point of death.
Agent Carruthers is the first to be affected, dying of old age at 28, but soon all CONTROL agents except Larabee are "old, withered and decrepit" The fact that Larabee is also the only one yet to have his new ID photo taken draws suspicion to the photolab and photographer Felix.
Felix is eventually tracked down at KAOS headquarters and arrested along with Lucheck and Lucheck's henchman Kinsky before he can complete the retouching of the CONTROL agents' photographs. An antidote to Dorian Gray is found in the form of a cleaning solution that removes the substance from the photographs and therefore the aging from its victims.
[Episode #123: "Age Before Duty".]