Selective color has a range very similar to that found under the "Color Range" selection tool, not as refinable as what you can do with a Hue/Sat layer, and I think it has a pretty sharp ramp off into the next color range. Download the Selective Color preset I have attached to this reply, open an image and create a Selective Color adjustment layer with the Saturation Mask preset. This won't make your image look very pretty, but if you go back through each color in the drop down and adjust the "Black" slider, always leaving it at -100 before going onto the next color, you can see clearly exactly what the Selective Color layer is adjusting for each color range.
