Hola!

You need to use the DXDraw.Surface.ColorMatch function to convert the TColor value to the value used by DirectDraw.
[pascal]DXDraw.Surface.Fill(DXDraw.Surface.ColorMatch(back _col));[/pascal]
However, the ColorMatch method is quite slow IIRC, so you might find it better to do the conversion only once and store it until required.