You can make either one colour in a Window/Dialog totally transparent by setting oWnd:nSeeThroClr := <nColor>. You can set this to any colour even at run time and even change during runtime. In the above case we set the background color of oDlg as RGB(1,1,1) and set it as transparent colour (nSeeThroClr ). To cancel, set this DATA to nil.
Alternatively, you can set oWnd:nOpacity := <nLevel> // 0 to 255
// 0 is fully transparent (no use at all) and 255 is fully opaque ( not at all transparent)
This make entire window / dialog transparent/translucent depending on the value of nOpacity.
You can set this value and change it at any time during runtime also. You can make fade-in fade-out effects too.
These two features are built in the Widow class (apply to all windows and dialogs) from FWH10.12.
These features internally use the same function that Antonio posted above : trans(oDlg).