The recent app list in Android 5.0 is much more vibrant than it was in KitKat with support for colored header bars, but not all apps are taking advantage of that. As of Chrome v39 (current beta release), developers can add an HTML tag to their site that does the same for Chrome tabs. It's not only the multitasking header—the tag also affects the status bar in Chrome. It's kind of crazy.
The process is super-simple. All you need to do is add a tag to your page's <head> with name="theme-color" set to any CSS color you like. For example, to set the header color to a nice relaxing indigo (as above on AP), you'd do <meta name="theme-color" content="#3F51B5">. There's also support for high-resolution favicons in Chrome v39, which will look nifty in the new app switcher.
This is only relevant on Lollipop, as earlier versions didn't have the same support for color usage. You also need to have tabs merged with apps in the Chrome settings menu. That makes each tab its own card, thus taking advantage of the individual colors on each page. Hopefully web devs start supporting this—it's neat. My only concern is that some sites might be a little obnoxious by turning your status bar some awful color.
[HTML5 Rocks – Thanks, Matthew]