Code:

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;