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.
- If developer options isn't there, you have to manually activate the feature. Follow the steps in this answer.
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.
