YouTube has recently updated the video page's design. Besides some obvious changes, like a slightly bigger player that lets you jump anywhere in the video, YouTube now lets you customize the player when you copy the embedding code. The custom players were already available for playlists, but now you can use them for individual videos as well. Just click on "customize embed" next to the generated HTML code and choose a theme. You could also add a border around the video and choose not to show related videos.
The code will probably include an URL that looks like this:
You can change the two colors from the URL (color1 and color2) to any value you want: the 216 web safe colors are a good start, but don't forget to replace "#" with "0x".
Here's a list of other YouTube parameters that could be useful if you want to manually edit the code (the default values are in bold):
* border (values: 0, 1) - if the value is 1, shows a border around the video
* rel (values: 0, 1) - if the value is 0, the player doesn't show related videos
* autoplay (values: 0, 1) - if the value is 1, the video starts to play automatically