I think lowering the Kr numbers lowers the amount of reflection for that color.

So "color Kr" [1.0 1.0 1.0] will be a bright white reflection
"color Kr" [0 0.1 0] will give a little bit of green reflection

Software: OS X 10.8 - Poser Pro 2012 SR2 - Luxrender 1.0RC3 - Pose2Lux
Hardware: iMac - 3.06 GHz Core2Duo - 12 GB RAM - ATI Radeon HD 4670 - 256 MB