How do I apply an effect to only one portion of a clip?

Hi,
In FCPX I want to darken the lettering on a sign that's in a clip without altering the rest of the image in that clip. I tried selecting the sign, but that cropped the sign and deleted the image around the sign. Do you know of a way to isolate one area within a clip and make adjustments on just that one area without deleting or altering the other portion of the clip?
Thanks for your help!

You can use keyframing to apply effects on one time range of a clip. You can also use a shape mask and/or color mask to constrain the correction to a physical region on the clip. A shape can also be keyframed to move. It does not necessarily require a lot of keyframes, FCPX will smoothly interpolate between just two:

Using FCPX shape and color masks for color. Remember each shape mask has an "inside mask" and "outside mask" setting:



FCPX color/luma keyframing: