You have probably encountered the Xcode syntax highlighting issue, very annoying. When this issue pops up you usually restart Xcode or clean the project and it’s all good. If that doesn’t help, you delete the derived data (Organized > Projects). If that doesn’t help, you restart your machine. If that doesn’t help, you have a problem.
None of the above solutions resolved the problem for me. It continued to plague me after I started using Xcode for a several minutes.
So what did it? Adding $(SRCROOT)/** to your project Header Search Path (Project > Build Settings > Header Search Path). Now delete your project’s derived data and it should work.