If you work with application that use a determinate theme, for example, opera and you run this, when you go to Editor\Themes tab, you change the Theme for BlackIsBlack theme and then you  select , for example Classic or Grayscale, the legend text becomes white. 
To reproduce the problem you can use a similar code as next: 
unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, TeeGDIPlus, TeEngine, TeeTools, Series, ExtCtrls, TeeProcs,
  Chart, TeeComma, StdCtrls, TeeEdit;

type
  TForm1 = class(TForm)
    Chart1: TChart;
    ChartEditor1: TChartEditor;
    Button1: TButton;
    procedure FormCreate(Sender: TObject);
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.FormCreate(Sender: TObject);
var Series1: TBarSeries;
begin
Series1 := TBarSeries.Create(self);
Chart1.AddSeries(Series1);
Series1.FillSampleValues(10);
Series1.ColorEachPoint := True;
ChartEditor1.Chart:= Chart1;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
ChartEditor1.Execute;
end;

And open the editor\Theme and change first the Theme to BlackIsBlack and then select Classic or Grayscale or Opera, or Web or TeeChartDefault. The Legend texts are white.