That's weird, I've heard of this happening to a few people, it's probably due to Instagram testing out new features, they often do this without announcing it to everyone. It's possible that you were part of a random group that got selected to try out the new purple theme, or maybe you accidentally tapped on something that triggered the change. You can try restarting the app or checking your settings to see if there's an option to switch back to the original theme, if not, it might just be a temporary thing and will go back to normal soon.