user6414: I exported a Flash movie with a black background to MOV, and converted the MOV to mp4 (H.264). In Quicktime player, the background of the MOV and mp4 are black. In native players in Firefox and  Chrome (desktop) and Safari and Chrome (ipad), the background of the mp4 is black. In IE11... it's dark gray. 
The actual gray is rgb 16 16 16, hex #101010. 
Any ideas?

user3295: The root cause is very likely the color space settings in your file. The video is also grey in Firefox on my machine but not in Chrome.
If you open your file in MediaInfo you can see this at the bottom of the video stream:
Color primaries                          : BT.601 NTSC
Transfer characteristics                 : BT.709
Matrix coefficients                      : BT.601

This should say BT.709 in all three categories. What is happening that the player is honoring the color spaces differently and mixing stuff up. This setting likely started out in the Flash export to mov. Handbrake shouldn't do that without explicitly giving it a command line option to do this.

user10669: I had this issue for a very long time in pretty much every program I played MP4 videos in. It turns out that by default, the Nvidia drivers don't actually have the full dynamic range enabled. I changed it, and tested it just to confirm that it really worked. Just open up the control panel, and change the dynamic range to "Full":


Regarding the current accepted answer: it doesn't seem to regard the fact that there will almost certainly be a large performance hit when disabling hardware acceleration, and is not a proper solution as a result. If it resolved the problem for you, it's pretty likely that just changing the driver settings (my answer) is far better than outright disabling hardware acceleration.

user6414: The solution was to turn off IE-11's "hardware acceleration" feature which (says here) "lets Internet Explorer move all graphics and text rendering from the CPU to the Graphics Processing Unit (GPU)".
I disabled that under Tools -> Internet Options-> Advanced by selecting "Use software rendering instead of GPU rendering".  It's pictured here
Then, after obligatory re-start (of IE), all the grays were black and colors bright in IE-11. 
Now, how I communicate this knowledge to IE users viewing my washed-out videos is a tough new question.

6414: <p>I exported a Flash movie with a black background to MOV, and converted the MOV to mp4 (H.264). In Quicktime player, the background of the MOV and mp4 are black. In native players in Firefox and  Chrome (desktop) and Safari and Chrome (ipad), the background of the mp4 is black. In IE11... it's dark gray. </p>

<p>The actual gray is rgb 16 16 16, hex #101010. 
Any ideas?</p>
 3295: <p>The root cause is very likely the color space settings in your file. The video is also grey in Firefox on my machine but not in Chrome.
If you open your file in <a href="http://mediaarea.net/en/MediaInfo" rel="nofollow">MediaInfo</a> you can see this at the bottom of the video stream:</p>

<pre><code>Color primaries                          : BT.601 NTSC
Transfer characteristics                 : BT.709
Matrix coefficients                      : BT.601
</code></pre>

<p>This should say BT.709 in all three categories. What is happening that the player is honoring the color spaces differently and mixing stuff up. This setting likely started out in the Flash export to mov. Handbrake shouldn't do that without explicitly giving it a command line option to do this.</p>
 10669: <p>I had this issue for a very long time in pretty much every program I played MP4 videos in. It turns out that by default, the Nvidia drivers don't actually have the full dynamic range enabled. I changed it, and tested it just to confirm that it really worked. Just open up the control panel, and change the dynamic range to "Full":</p>

<p><img src="https://i.imgur.com/I4wtPTb.png" alt=""></p>

<hr>

<p><em>Regarding the <a href="//video.stackexchange.com/a/12801/10669">current accepted answer</a>: it doesn't seem to regard the fact that there will almost certainly be a large performance hit when disabling hardware acceleration, and is not a proper solution as a result. If it resolved the problem for you, it's pretty likely that just changing the driver settings (my answer) is far better than outright disabling hardware acceleration.</em></p>
 6414: <p>The solution was to turn off IE-11's "hardware acceleration" feature which (<a href="http://support2.microsoft.com/kb/2528233" rel="nofollow">says here</a>) "lets Internet Explorer move all graphics and text rendering from the CPU to the Graphics Processing Unit (GPU)".</p>

<p>I disabled that under Tools -> Internet Options-> Advanced by selecting "Use software rendering instead of GPU rendering".  <a href="http://support2.microsoft.com/Library/Images/2685831.png" rel="nofollow">It's pictured here</a></p>

<p>Then, after obligatory re-start (of IE), all the grays were black and colors bright in IE-11. 
Now, how I communicate this knowledge to IE users viewing my washed-out videos is a tough new question.</p>
