Check out some interesting strings in the Android 9 source code hinting at enhanced Always On Display feature. The code were displayed in source website maybe a future Android Pie update, you will be allowed to set a wallpaper to the Always On Display and it goes without saying, it will turn it into black and white to consume less power.
According to the first commit, for a wallpaper to show on the Always on Display it has to define the “supportsAmbientMode” attribute as “true” in its android.service.wallpaper metadata. You can view an example of how that’s done in the sample live wallpaper app that Google uploaded to AOSP. The second commit states that, by default, the wallpaper on the Always on Display will be hidden after 1 minute. The system transitions from the wallpaper background to a black background with a cross-fading animation that lasts 400 milliseconds. Thus, it doesn’t look like Google’s take on the feature will be as powerful as Samsung’s which has not only supported static wallpapers for some time but even recently started supporting animated GIFs of a few seconds in length- XDA Developers mentioned.
The code also reveals that the cross-fading animation is 400 milliseconds long and the wallpaper will disappear after one minute.