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.

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.