How to fix an iPhone with half a screen showing a different color?
I would also try a full factory reset, like IconDaemon suggests. If that doesn't improve your situation, you have the following choices:


Go to an official Apple store or a premium service provider to exchange your screen (this will cost you between 100 and 300$ depending on your location)
Go to an iPhone doctor or clinic which replaces the screen with a cheap aftermarket china one (Be very careful with this, most of the screens have a very bad quality and you can't get support from Apple afterwards. To check, if a screen is genuine, there should be no yellow glare and you can press firmly on the glass without damaging the display)
Buy a broken iPhone online and change the screen yourself as this tutorial on ifixit states: iFixit - iPhone 6 Repair