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

include/commdlg.h

diff --git a/include/commdlg.h b/include/commdlg.h
index 43216ef..adf8ac7 100644
--- a/include/commdlg.h
+++ b/include/commdlg.h
@@ -123,7 +123,7 @@
 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 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

diff --git a/programs/cmdlgtst/cmdlgtst.c b/programs/cmdlgtst/cmdlgtst.c
index 442153a..864f30d 100644
--- a/programs/cmdlgtst/cmdlgtst.c
+++ b/programs/cmdlgtst/cmdlgtst.c
@@ -152,7 +152,7 @@
 
 	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;