H.264 versus Apple Prores 422HQ

I am finishing an HD project on FCP 7 and when I create a finished movie with 1920x1080 in H.264 the film clips match the color and luminosity of the FCP timeline and look great. When I create the same movie in Apple Prores 422HQ almost all of the clips are dark ... some so dark as to be totally unacceptable. As I may have to submit this film to a distributor in Apple ProRes 422 HQ, is there something I am doing incorrectly?

FCP 7
OS X10.6.8
Macpro Intel