With both iOS and Android supporting dark mode, some interesting analyses on energy consumption savings of dark mode is now available.
Personally, the most interesting takeaway is that dark grey (with significant accessibility improvements) is about as battery efficient as ‘full black’ (which still has some accessibility concerns).
The fundamental cause seems to be the energy curve of OLED screens
At the recommended 0.3% of luminance of white on a Oneplus Pro screen
Now, barring unique display profile / configuration of the OnePlus Pro screen, this is a good scientific observation (also backed up by 2 real world measurement experiments).
So, wonder why Apple is at least marketing pure black dark mode in the iOS 13 screenshots.