This message is sent to a dialog box before Windows draws the dialog box. By responding to this message, the dialog box can set its text and background colors by using the given display device context handle. The DefWindowProc function selects the default system colors for the dialog box.
The system does not automatically destroy the returned brush. It is the application's responsibility to destroy the brush when it is no longer needed.
The WM_CTLCOLORDLG message is never sent between threads. It is sent only within one thread.
Note that the WM_CTLCOLORDLG message is sent to the dialog box itself; all of the other WM_CTLCOLOR* messages are sent to the owner of the control.
If a dialog box procedure handles this message, it should cast the desired return value to a BOOL and return the value directly. If the dialog box procedure returns FALSE, then default message handling is performed. The DWL_MSGRESULT value set by the SetWindowLong function is ignored.