Without any info about render settings, your system's color management and so on nobody can tell you much. screenshots would also apparently tell us a lot more than generic descriptions. The general gist likely is that you're working "blind" - using a specific monitor profile/ mis-tweaked monitor without compensating for it by using color management plus on top of course you may simply not be aware how some CoDecs limit color ranges. anyway, more info is required.
Mylenium