Google introduced the power saving mode in the last developer preview, and it seems to work the same this time with one notable exception—orange. Lots of orange. When you activate power saver mode, your status bar and nav bar will turn orange and stay that way until you shut off power saver.
When you activate this mode, the screen brightness is reduced, animations are turned off, background data is (mostly) disabled, and vibration is off. Basically, the device is less fun to use, but it runs longer. Google wants to make really, really sure that you remember you have battery saver on, I guess. The notification icon isn't enough, so your system bars are orange.
It's orange and completely solid everywhere, even in apps that have transparent bars or make use of the new header color. That can lead to some clashing hues. There doesn't appear to be any way to disable this extraordinarily loud visual cue, so you're probably going to have to live with it.