Easiest solution:

1. Drop the Color Corrector (Secondary) FX onto the clip.
2. Click eyedropper labled the "Select effect range" and, in the preview window, click the color you want to keep. Or, if you need a range of colors or hues of a color, draw a box around them. Do your best not to include any colors you don't want to keep.
3. Check the box labedl "Invert mask"
4. Drag the slider labeled "Saturation all the way down.

That'll get you started. The color you picked will be untouched, but everything else will be black and white. From there, you will probably need to fine-tune the Color Corrector (Secondary) settings. The sliders on the top half adjust the effect itself, so you probably won't need to do anything with that. Where it's at is the sliders at the bottom, which give you more control over what range of pixels are affected.

Keep in mind that this affects colors, not objects. So if the range of colors you want to keep happens to also appear in an object or area that you want to be black and white, you'll need to use some other tools. For example, you can duplicate the track, make the bottom track all black and white, and use bezier masking on the upper track to cut out the problem area. But then you may need to do some fancy keyframing to keep that mask from ending up over your keep-it-color area.

Rob