I have created a "Conditional Hue/Saturation" effect plug-in for PDN. It basically just goes through the image and analyzes the hue and saturation values of the pixel, checks it against the range specified by the user, and then takes the appropriate action, which may be a hue shift, saturation shift or hue fade. While not much of a technical achievement I've found it useful for quick changes that could be done "by hand" but are a one-pass action with this effect.   Below are some befor