Load a ProRes clip into the Viewer in Final Cut Pro and it looks a little dark. Hit play and it brightens (gamma) until you pause, then it goes a little dark again.
I'm not sure why this happens but I see it regularly with ProRes clips. Some say it's the data rate being lowered for slower drives, others say it's the type of compression the timeline is using (highlight the sequence in the Browser, hit Command + 0 (zero) then look at QuickTime Video Settings in the lower left).
There's a brief discussion of it here on the Apple boards.
For me the difference in a clip that does it seems to be the size.
A ProRes clip which is 854 x 480 does have the brightness playback issue.
A ProRes clip which is 720 x 408 does not have the brightness playback issue. (Inconsistent though; some clips have the issue while others do not)
A ProRes clip which is 720 x 480 does not have the brightness playback issue.
That's the only difference I can determine between the two clips.
Playing the clips in QuickTime Player does not show a gamma shift.
|Gamma while Playing in Viewer|
|Gamma while Paused in Viewer|
I don't know a solution to this and really, it's just an annoyance overall; but it's good to be aware of it.