One of the coolest things about Windows Phone is the accent colors. Not only do they determine the color of your Start screen tiles, but they also show up all over the OS, and even in 3rd-part apps. A recent video tour of Windows Phone 8 shows a new place that the accent colors appear.
The video has since been removed, but the screenshot above still shows the new feature. As you can see the keyboard uses the accent color to show what key you have pressed. In the past the pop-up key has just been the same color as the keyboard. It’s a very small change, but it shows the attention to detail Microsoft is putting in WP8.