I have just upgraded to the new Xcode 4 and the code is not colored has it ought to be.
For instance, the string NSString is not colored in my custom code, but when I switch to Apple's code (NSString.h for example) everything is well colored.
How can I fix that?
It's a known bug with the latest XCode. This happens with some projects which are migrated from XCode 3.X to the new version. For some people it seems to help to go to the organizer, and in the project tab delete the derived data for the project where code sense does not work correctly.
If you have a small project, it might also help to create a new project in XCode 4 and import the files from the XCode 3 project.
Update: XCode 4.0.1 made the situation a bit better, at least in some of my projects I have syntax coloring and code sense back. But it's still far from fixed.
Update 2: XCode 4.0.2 did not change much. It seems that the problem is related to subprojects, specifically static libraries. According to comments of this blog post some people were successful by changing header search paths from relative to absolute paths, e.g. instead of Foo/Bar use $(SOURCE_ROOT)/Foo/Bar. This together with switching all projects to XCode 3.1 format and to use LLVM 2.0 fixed a lot for me as well. It's not perfect yet, but usable.
Update 3: After converting the sub projects to independent projects and putting them in a workspace (aka the XCode 4 way) I have now full syntax highlighting and code completion back.
My current environment is now a XCode 4 workspace with each projects being 3.1 compatible (as opposed to the default which is 3.2), LLVM GCC 4.2 (system default) and the header search paths are still absolute (using $(SOURCE_ROOT)/.../).