A lot of video games have filters now for things like friendly/enemy tags and such.
If you don't have developer options enabled you need to tap on the "Build number" 10 times in the "About phone" settings to enable it on Android.
Separately, I've got a "Night Mode" setting, which lets me set a schedule and a color temperature to use.
What a dream. May I ask what Android version you're on? On 8.1.0 it's not there.
EDIT: will not work on 5X, only on Pixel devices...
Why do grey pixels consume less power than red pixels of the same brightness?
In fact, assuming (maybe wrongly) that your phone screen's colors are created from RGB, CMYK or some other combination of primary color 'dots' in each pixel, grey would require lighting all the dots in a pixel, while 'redscale' would limit power usage to 1/3 or 1/4 of the dots. I realize there are many other issues potentially involved, but perhaps someone could explain.
EDIT: Sorry, the correct terminology of 'dots' and pixels is lost in my brain someplace and I don't have time to find it.
What takes more power depends on the hardware. An OLED display would take more power to light up a pixel than to keep it dark. An LCD display would typically take slightly more power to show a dark color. The backlight is always shining (which takes most of the power), but darkening an LCD pixel takes more power than just letting the light through does.
Maybe the actual power savings would come from lighter rendering calculations (only having to calculate a single color channel)?
The backlight has to be strong enough to supply the highest R,G, or B value, while the lower values have light blocked off (by polarization) and turned into heat.
When the LCD display shades of grey, the RGB values are more likely to be in tandem, so maybe you can get the same final brightness coming out with fewer photons being overproduced at the start.
It's usually under Developer Options.