Ok nevermind about the macros, I found a better solution:

Code:

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typedef DWORD COLORREF; //unnecessary, but for being more similar to the msdn examples
typedef struct _CHOOSECOLOR{
    long lStructSize;
    long hwndOwner;
    long hInstance;
    char rgbResult[4];
    COLORREF *lpCustColors;
    long Flags;
    long lCustData;
    long lpfnHook;
    char* lpTemplateName;
}CHOOSECOLOR;
DWORD __stdcall ChooseColor  (DWORD); //function prototype
 

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If the rgbResult member is defined as a char array, the bytes of the rgb value structure (0x00BBGGRR or sth. like that) will be seperated automatically. No need for the macro functions then.