We have known for a long time that apps in dark / night mode can extend battery life on smartphones with OLED screens. It's true on Android, and it's true with the iPhone. This is because the individual pixels have to do less work on the dark areas of the screen and do not use practically any juice when they display true black. While SlashGear filming, Google repeated this week during his Android Dev Summit this week, showing several slides that compare the power of different colors. (The company used its original Pixel smartphone for these data points.)
You can see that white far consumes more energy. This has led Google to recognize that the importance of white in its apps and Android style guidelines is, well, less than ideal. It is everywhere and is not changing with the renewed Material Design by Google.
Fortunately, the company seems to recognize the value of the dark mode. The YouTube and Android messages already exist, and Google is also introducing the feature in the Telephone app and testing it on the Google Mobile feed. Android can also be set to a dark theme for the shortcut menu and the app drawer, but Google has not yet arrived to add a system-wide night mode. (This is something that Samsung intends to do with its new UI One.)
As an example of the dark mode that comes to saving the battery, just look at the huge difference below, where the Pixel is set to 100% brightness. The energy savings of the dark mode are obvious. I tend to stick more often to the traditional look of these apps, because I find the white text on black a bit harsh in my eyes, but it's nice to see that Google is recognizing the value of increasing the white level. Make it a choice wherever it makes sense.