Download and colorDebug blog:
http://3desque.wordpress.com/2011/03/23/colordebug/

This Nuke plugin will help you pin-point color values outside or inside the defined boundaries.

Knobs description:
Lower: Lower boundary value
Upper: Upper boundary value
InsideBoundaries: Displays the color range between Lower and Upper
LowerColor: Color to use for everything below Lower
UpperColor: Color to use for everything above Upper
InsideBoundariesColor: Color to use for everything between Lower and Upper

Source code included.
Compiled for Nuke6.1->linux64 and Nuke6.2->win32, win64

Enjoy and drop me a line if you have any questions.