EyeonTips:Manual/Tool Reference/Creator/Fast Noise
From VFXPedia
Gradient mapping
You can use the Fast Noise tool to do gradient mapping (changing a pixel's color based on its brightness), with a Bitmap Mask connected to the Noise Brightness input.
Connect your source tool to the Bitmap Mask (set to Luminance), connect that to the FN's Noise Brightness Map input, set Noise Detail to 0, turn up Brightness, and start playing with the Gradient color controls. There's more info and a macro to download here.