Are you trying to make footage shot in daylight look like it was shot at night? If so, try using the Video FX to first darken your footage, then use the Color Balance effect to add more bluish tones to the scenes. I find it works better to do this in your camcorder as you shoot - if your camcorder has manual controls, reduce your exposure to darken your footage, and set the white balance to the indoor setting to add bluish tones. It can be quite effective. Try not to have any of the sky in your scene, however, as daylight sky is too bright to darken enough to look like true night.