Why are the headers files different? What does it mean?
Black means: The file shall be compiled. Grey means: Do not compile the file.
For C++ implementation files (e.g.
*.cpp) the default option is to compile the file until the user explicitly turn this off.
For C++ header files (e.g.
*.h) the default option is
not to compile the file until the user explicitly turn this on, which is the case usually only for pre-compiled header files a.k.a. "PCH".