Although we’re very excited about this change, which comes after months of design, research, and experimentation, we realize that some embedders would prefer a lighter color to match a lighter webpage, so we’re also announcing a few other themes that we’ll be making available. We also realize that some embedders would prefer something more mild than our use of red, so we’re happy to introduce a "desaturated" color option without any bright colors at all. (Note: Using the desaturated color disables the modestbranding option.)
To pick an alternate theme, pass in values for the "theme" and "color" player parameters. For instance, if you’d like to keep the traditional colors, you can pass parameters such as:
<iframe width="560" height="349"Here are all the new themes, including the new default:
If you have any questions about this change, you can ask them on our forum.
—Shannon -jj Behrens, YouTube API Team