Since most all of the video hardware is set up to allow access by only one app at a time, it can get complicated when trying to share. Most software these days at least trys to be a good citizen, but it doesn't always work as planned.
For non-FireWire/DV video hardware (DeckLink, Kona, etc.), then you just need to turn off previewing in FCP before firing up the Color Finesse UI. Fortunately, this is just a single keystroke (shift-F12) to toggle it on and off.
FireWire/DV is a problem, because of a bug in the FireWire software. FCP tries to release the connection, but even though it does everything right, it doesn't really get released, which causes Echo Fire (or any other software) to get an error when it tries to get a FireWire connection. I don't know of a workaround to this.
Bob Currier
Synthetic Aperture