Easy
// Create a popup GDImage overlay window
HWND CreateSplitter(IN HINSTANCE hInstance) {
RECT lpr; GetWindowRect(gP.hGL, &lpr);
DWORD dwStyle = WS_POPUP | WS_VISIBLE;
DWORD dwExStyle = WS_EX_TOOLWINDOW;
HWND hWnd = CreateWindowEx(dwExStyle, GDImageClassName, NULL, dwStyle, lpr.left + Width(lpr), lpr.top, SPLITTER_WIDTH, Height(lpr), gP.hMain, 0, hInstance, NULL);
if (IsWindow(hWnd)) {
//Setup the custom color background here
ZI_SetProperty(hWnd, ZI_GradientTop, ZD_ARGB(128, 0,255,0));
ZI_SetProperty(hWnd, ZI_GradientBottom, ZD_ARGB(128, 0,255,0));
ZI_UseWinLIFTbackground(hWnd, TRUE, FALSE);
The beauty of this API is that it would work also with a complex background, if you use it in the Callback WM_MOVING section