There have been numerous complaints that the FZ10 has a dark electronic viewfinder (EVF) in low-light conditions. The darkness is even worse in full manual mode. In manual mode the camera attempts to show just how dark the resulting photograph might be (by reducing the aperture diameter, rather like pressing a depth-of-field button on a single lens reflex camera) all the time, not just when the shutter release is half-depressed.
Using the rear-panel LCD rather than the EVF may help a little (FZFun, dpreview, 8 March 2004). but in my own experience, the EVF looks a little brighter. Perhaps everyone's FZ10 is a little different, so experiment and find out what works best for you!