You use the three way color corector (or just the color corrector filter)
in FCP 5 the limit effect is hidden - it's a twirl down arrow at the botton of the filter.
Use the eyedropper to select the color you want to keep
Click on the key symbol to see exactly how much of the color you actually selected.
You can expand the range of color using the sat and luma and color timelines to grab more of the color....your key mattte will expand as you do this...click the key again to get your final mat...drop the saturation and that color will disappear...
If you want to only keep that color....click the bottom symbol under the key (not exactually sure what that is...a distorted square? Parallelogram? ) it will invert your picture and keep only the color you selected...and with the saturation dropped to zero - turn the rest black and white.
Enjoy your pleasantville experience.
Andy Field