I'm having a similar problem so I sort of wanted to piggy back off this thread instead of posting a new one.
I've cut a sizzle reel with mixed media pulled from the web and some shot with a canon 5D/7D. The media has all been transcoded to apple pro res 422/23.98. It looks fine when we playback in FCP 7 and when we export a full res QT but as soon as I try to compress for web, some of the clips have gamma fluctuations. The problem isn't consistently on the same clip or same places but changes every time. We've tried exporting through compressor, compressing with mpeg streamclip, compressing with Quicktime 7 … and still have export issues.
Anyone have any ideas?
Mac OSx 10.6.8
Memory- 8 GB RAM
Processor- 2 x 2.26 GHZ Quad
The FCP 7 editing interface isn't designed to be color accurate. The Viewer, the Canvas...won't show you accurate color. They are there to show you what you are doing, that's it. For an accurate representation of what you see in FCP 7, you need an IO device that you can hook up to an HDTV...something like the Decklink Ultrastudio Mini MOnitor, or AJA T-Tap.
FCP-X...the latest version of FCP, IS designed to display the proper colors. Why wasn't FCP 7? Because it was designed with playing on TVs in mind...either over the air or from DVDs or tape. FCX knows that most of the video made will be viewed on computer displays, so it was designed accordingly.
And anytime you compress to another format, you'll lose some color information and the clip won't look exactly the same as the full res original. This is how compression works...information is lost to save space.
Gamma shifts are a well documented problem with Apple's first party software/codecs. It's been a while since I've run into the problem so I don't remember the various work arounds but if you Google "Final Cut Pro" and "gamma shift" you'll get lots of hits.