Grayscale mode is a new accessibility feature available in iOS 8. It's a useful option for anyone who is color blind. If you enable grayscale it will replace all colors with a range of gray shades from white to black.
How to enable grayscale mode in iOS 8
1. Go to Settings app on your iPhone or iPad.
2. General.
3. Accessibility.
4. Turn On the option for Grayscale.