I’m pretty sure those savings would apply only on pitch black interfaces on OLED screens, IPS panels do not use less energy by displaying black, unless is the only color on screen.

If a UI is dark grey (like the Nestle one in this post screenshot), all of the diodes on an OLED screen are lit up, so there shouldn’t be any savings whatsoever.

Also, we’re far from having dominance of OLED screens in the smartphone market, which would be required for your scenario to apply.