Depending on your system, video may look muddy, with black not really black. Usually this happens if you use Windows Media Player, or under certain configurations with MPC-HC.
Here's an example:
The video is supposed to look like this:

But it looks like this, with a very muddy and washed out color:
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Solution? You'll have to use Media Player Classic. Can't change this under Windows Media Player, unfortunately.
Here are the steps to do this:
1. Under MPC-HC, click View > Options
2. Click "Output", which is under "Playback" options, at the left sidebar.
3. To the right, you'll see a lot of radio buttons. Change the DirectShow Video output to "Haali Renderer **"
4. Click "Ok"
5. You're done!
If you have been using VMR7 or VMR 9 renderless modes before, you will no longer be able to quickly change Audio and Subtitles by right-clicking the video (both options are greyed out). If you want to change them by right-clicking, you'll have to hover over "Navigate". Then you'll be able to effect changes there.
Alternatively, just click Navigate in the menu bar. Or, depending on how you configured it, click the FFDshow/Haali Media Splitter at the taskbar notification area.
If you wish to undo, just change the DirectShow Video output back to what it was. Chances are it would be "Overlay Mixer" or one of the VMR renderless modes.