//проверял только в Delphi 6 + нужен "uses RichEdit"

procedure SetRichEditSelectionBGColor(ARichEd: TRichEdit; ABGColor: TColor);
var
   recCharFormat: TCharFormat2;  
begin
   //BG-Color
   FillChar(recCharFormat, sizeof(recCharFormat), 0);
   recCharFormat.cbSize := sizeof(recCharFormat);
   recCharFormat.dwMask := CFM_BACKCOLOR;

   if ABGColor<>clNone then
      recCharFormat.crBackColor := ColorToRGB(ABGColor)
   else
      recCharFormat.crBackColor := ColorToRGB(ARichEd.Color);

   ARichEd.Perform(EM_SETCHARFORMAT, SCF_SELECTION, LParam(@recCharFormat));
end;