[roman]

Código Delphi [-]
var
  Form: TForm;

begin
  Form := CreateMessageDialog('Hola mundo', mtCustom, [mbOk]);
  Form.Color := clRed;
  if Form.Controls[0] is TLabel then
  begin
    TLabel(Form.Controls[0]).Font.Color := clGreen;
    TLabel(Form.Controls[0]).Font.Style := [fsBold];
  end;

  with Form do
  begin
    try
      ShowModal;
    finally
      Free;
    end;
  end;
end;

ShowMessage usa internamente a CreateMessageDialog.

// Saludos