My iphone fell and the screen went pink in color. What do I get back to its original color?

Try restarting it. But it's most likely damaged. And since it's from a drop/fall, an AppleCare warranty - even if still in effect - won't cover you.

https://discussions.apple.com/thread/757687

Make an appointment at your nearest Apple Authorized Service Center or an Apple Store. Get an estimate for the repair, then decide if it's worth paying to have the repair done.

If you dropped it and it is damaged, you can only solve the problem by buying a new one.