You can see in my attachment the slight color difference in video output between Flash and HTML5. The HTML5 (in FF 38) definitely has a gray cast to it.
This is made more noticeable because the first frame of video shown before the videos plays, does not have the cast and is nice and bright. As soon as the video starts, it changes to gray. I checked the values with a color picker, white in Flash has a value of 253, while white in HTML5 has a value of 235.