You can use the leave color filter which will allow you to target a specific color, and desaturate the other colors…this is pretty much what you said you wanted, but there is also a better way which allows for more flexibility…
I use the change color filter.
I start by duplicating twice my layer, then I apply the change color filter to the uppermost layer. I select the color, then select color correction matte from the view pulldown. Next I use this track as a track matte for the 2nd layer using luminance as the type of track matte. Now I add the Hue/Saturation filter on the bottom layer and desaturate….so basically this gives you the same look as the leave color……the big advantage to this method is that you can now tint the bottom layer or adjust it’s levels etc without affecting the color that you left….and for interesting reaults, you can transfer mode the colored layer back onto the black and white layer.
Hope this helps
Mark
Mark Harvey
Senior Editor
Le R