I triple checked but yes, CHOOSECOLOR.hInstance is an HWND...

include/commdlg.h

@@ -123,7 +123,7 @@ typedef UINT CALLBACK (*LPCCHOOKPROC) (HWND, UINT, WPARAM, LPARAM);
 typedef struct {
 	DWORD		lStructSize;
 	HWND		hwndOwner;
-	HWND		hInstance;
+	HWND		hInstance; /* Should be an HINSTANCE but MS made a typo */
 	DWORD	        rgbResult;
 	LPDWORD         lpCustColors;
 	DWORD 		Flags;
@@ -136,7 +136,7 @@ typedef CHOOSECOLORA *LPCHOOSECOLORA;
 typedef struct {
 	DWORD		lStructSize;
 	HWND		hwndOwner;
-	HWND		hInstance;
+	HWND		hInstance; /* Should be an HINSTANCE but MS made a typo */
 	DWORD	        rgbResult;
 	LPDWORD         lpCustColors;
 	DWORD 		Flags;

programs/cmdlgtst/cmdlgtst.c

@@ -152,7 +152,7 @@ void mwi_Color(HWND hWnd)
 
 	cc.lStructSize = sizeof(CHOOSECOLOR);
 	cc.hwndOwner = hWnd;
-	cc.hInstance = g_hInstance;
+	cc.hInstance = (HWND)g_hInstance; /* Should be an HINSTANCE but MS made a typo */
 	cc.rgbResult = RGB(0,0,0);
 	cc.lpCustColors = cc_cr;
 	cc.Flags = 0;