Books in the Colorless series

"A massive solar flare erased nearly all color from the surface of the Earth, and the mutated descendants of the human race exist in a state of slow decline. What little color remains now holds a strange and seductive power. Some people, like the lone-wolf professor Avidia, try and understand it. Others, including the cult-like Iodopsy Order, use it for power and control. When an Order assassin draws innocent bystanders into their rivalry, it'll take everything Avidia has to save their lives...and his own!" --provided by publisher.