We've discussed in the past how blue light from your electronics can have bad effects on your bedtime. Google recognizes this as an issue as well, and has added a new color mode to their Play Books app.
The new mode, called Night Light, will use hues of amber based on available natural sunlight to filter blue light, and therefore reduce eye strain. In the early part of the day, book pages will appear in the color setting you've chosen. As the day goes on, the pages will become the warm amber color.
This feature is still rolling out to users, but if you want to skip the wait, you can install a copy from APKMirror. This site compares cryptographic signatures to make sure you are getting a legit copy of the app. After your download finishes, tap the notification and install the new version of Play Books.
Upon opening your first book after updating, you will be notified of the Night Light mode. However, if you miss this notification, you can easily enable it by tapping the edge of the page > A icon > toggle Night Light mode.
If you use the inverted color scheme (black background with white text) for your books, Night Light mode will still work by adjusting the font color to increasingly darker shades of amber.
As an added bonus, AndroidPolice noted that this version of Google Play Books comes with a setting to receive notifications when an author in your library publishes new content. This is enabled by default and includes all authors, but you can disable it in Settings > Notifications (at bottom) > Author (check box).
If you enjoy the way Night Light mode works, you may also be interested in checking out other apps that use this same color-shading method across your entire device.
(Via Android Official Blog)