Have you tried restarting Xcode? I'm using an older version than you are but it behaves weird too sometimes (such as autocompletion randomly stopping). I just save my project, quit xcode and reopen it and it is just fine.
Xcode > Preferences > Fonts & Colors. What theme are you using? Try a different one. Low Key and Printing are both terrible themes for noticing syntax coloring. I like Default, personally.
Hmm, a .pch may cause problems, but if it is misplaced, your project won't build at all. Have you modified your .pch in any way? If you have, check the order in which you are importing your headers.
Also check the order of compile sources. Generally, classes that are imported in other classes should be at the top.