Ok., I'm going to open a can of worms AND possibly confuse everyone...
On my Galaxy S7 I somehow figured out how to configure the home button to give me a magnifier option and/or a greyscale option if I click it three times in succession.
So triple click home button and a menu comes up with "magnifier" and "greyscale".
Ok the window says "Direct Access"...
Found it...
Accessibility >Direct Access
Toggle it to on, select what you would like to be accessible in the menu when you triple click the home button. Now I don't know that greyscale actually does anything for battery life or not...but it's an interesting thought. Other options in that menu are TalkBack, Voice Assistant, Universal Switch, Magnifier Window (basically same functionality as what you get with the S pen, but no pen needed), Negative Colors and Grayscale. There is also color adjustment and interaction control which require additional settings to be set in Visual and Dexterity accessibility options.