The Marshmallow update for Samsung phones (mine is a Galaxy S5, but the Note series has the same problem) seems to have removed the ability to turn on grayscale mode when activating power saver mode.
Is there a way to re-enable this feature?
Samsung removed the feature from Power Saving, but you can still activate grayscale mode in the developer options.
Go to your Settings and into the Developer Options.
Scroll to the
Hardware-accelerated rendering section, and click on
Simulate color space. Then select
Monochromacy. Your phone is now in grayscale mode.
This color space option also exists on other phones.