uses
 RichEdit;
procedure RE_SetSelBgColor(RichEdit: TRichEdit; AColor: TColor);
var
 Format: CHARFORMAT2;
begin
 FillChar(Format, SizeOf(Format), 0);
 with Format do
 begin
  cbSize := SizeOf(Format);
  dwMask := CFM_BACKCOLOR;
  crBackColor := AColor;
  Richedit.Perform(EM_SETCHARFORMAT, SCF_SELECTION, Longint(@Format));
 end;
end;
// Example: Set clYellow background color for the selected text.
procedure TForm1.Button1Click(Sender: TObject);
begin
 RE_SetSelBgColor(RichEdit1, clYellow);
end;