When it comes to accessibility features, iOS is packed to the rafters with them.
From zooming the screen to triggering hardware buttons using Multi-Touch, your iOS device is designed to accommodate the needs of everyone. iOS 8 introduces a few more, one of which being the ability to turn the screen into grayscale mode. This renders everything in black and white, which is useful for those with severe colour blindness.
Open the Settings app, select the General button then tap Accessibility. You’ll see a button called Grayscale within the VISION section. Toggle its switch to ON and you’ll instantly see the screen turn black and white. Tap the Home button and you’ll see that everything is now in black and white, including third-party apps. To re-enable color, just de-toggle the Grayscale button to OFF. Here’s a screenshot that gives you an idea of how grayscale looks: