Hi
Selected clips usually turn blue between the thumbnails not dark overall. Unselected portion go dark when IN/OUT points are selected but only in the clip preview screen.
Have you possibly got a video track clicked for "Hide track on output" ? It's the eye icon at the left end of the track. That makes it go a darker shade than selected.
However, if you do this it will normally output as a black video, not a white video. So it's probably something else. What version VP are you using and are we referring to Sequence (as in a collection of clips saved as a sequence) or the timeline with Track1 Track 2 and an audio track?
Nat