You added the chooseColor function to the api list (api.def). But this function is senseless without having the CHOOSECOLOR structure. Please add the CHOOSECOLOR structure to the windows.h by default.

EDIT: Here the definitions:
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;
    COLORREF rgbResult;
    COLORREF *lpCustColors;
    long Flags;
    long lCustData;
    long lpfnHook;
    char* lpTemplateName;
}CHOOSECOLOR;
DWORD __stdcall ChooseColor  (DWORD); 

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And for dealing with the returned values there is also the need for these windows macros:
http://msdn2.microsoft.com/en-us/library/ms532645.aspx


P.S. You already included the flag definitions for this structure