I’ve noticed over the last decade or so that videos played in quicktime are often (if not always) washed out and a little desaturated. If you’re watching a long video you could get a much crisper image spending just a few seconds with the A/V controls beforehand. Just go to Windows->Show A/V Controls (on a Windows machine). The two suggestions I would give are to bump up the Contrast and Color sliders ever so slightly. You should adjust the contrast setting on a frame of video with a good amount of black, and you should adjust the color setting on a frame with, go figure, lots of color. Have fun.
P.S. The same is true for video in iTunes.