From the manual:
“
Final Cut Pro includes an automatic color-balancing feature. When you use the color-balancing feature, Final Cut Pro samples the darkest and lightest areas of the image’s luma channel and adjusts the shadows and highlights in the image to neutralize any color casts. In addition, Final Cut Pro adjusts the image to maximize image contrast, so that the shot occupies the widest available luma range.
”
I love that: *When you use..." -- apparently most people would rather try and do their own balancing the FCP7/Color way...
If you're white balancing your camera, that's one thing. You only need a "white card".
IF you're color balancing in FCPX, you can't do better than using this automatic balancing feature. Apple has left it turned off by default, because [again] FCPX's color board is NOT ABOUT balancing color -- it's about grading color (or colorizing the image for effect.) BUT - the balance feature IS THERE WHEN YOU NEED IT. Turn it on! IT IS PERFECT. (You *should* analyze clips for color balance either on import or selectively in the Event browser first... but even if the footage isn't analyzed first, FCPX does and *outstanding* job of balancing color.)