How can I export a Video without making it darker than the original file?

Hi there!

I'm working on a footage shot with the Canon 5D and since the first step in the post I exported the raw footage with Apple ProRes 422 (HQ) and worked with it in After Effects and later on FCP and then the usual Color and Compressor. But somehow every time I exported it, it got darker and darker... I tried using Animation codec instead of Apple ProRes 422 (HQ) and it didn't get dark but the files are HUGE and in the end I have to export the video in H.264 for internet. Does anybody out there have any idea what should I do and which compression should I use and how?

Thanks a lot!

Davi