[Bryan Harris]

Find the clip you need to adjust and reveal in event browser (shift F). Take the clip from the event browser and lay it on the next video track over the original in sync. Effectively you have over-layed the same clip on itself. Now set colour change extremes in one clip, then the other. Use opacity keyframing to switch from one to the other.
I worked this out when needing to exposure grade across a shot with auto exposure hunting.