Как выбрать цвет пользуя TTrackBar?

Drop three TrackBars on a form. Set Min to 0, Max to 255. Drop a TImage on the form. Then try this code:

{ ... }
var
  Form1: TForm1;
  MyColor: LongWord;
  RedColor: LongWord = $00000000;
  GreenColor: LongWord = $00000000;
  BlueColor: LongWord = $00000000;
 
implementation
 
{$R *.DFM}
 
procedure TForm1.FormCreate(Sender: TObject);
begin
  DoImageFill;
end;
 
procedure TForm1.DoImageFill;
begin
  MyColor := RedColor or GreenColor or BlueColor;
  Image1.Canvas.Brush.Color := TColor(MyColor);
  Image1.Canvas.FillRect(Rect(0, 0, Image1.Width, Image1.Height));
end;
 
procedure TForm1.RedBarChange(Sender: TObject);
begin
  RedColor := RedBar.Position;
  DoImageFill;
end;
 
procedure TForm1.GreenBarChange(Sender: TObject);
begin
  GreenColor := GreenBar.Position shl 8;
  DoImageFill;
end;
 
procedure TForm1.BlueBarChange(Sender: TObject);
begin
  BlueColor := BlueBar.Position shl 16;
  DoImageFill;
end;
 
end.

Each color value ranges from 0 to 255. Set the three trackbars with this range. You can use the RGB function to create a color from these values.

{ ... }
type
  TForm1 = class(TForm)
    redTrackBar: TTrackBar;
    greenTrackBar: TTrackBar;
    blueTrackBar: TTrackBar;
    Panel1: TPanel;
    procedure blueTrackBarChange(Sender: TObject);
    procedure greenTrackBarChange(Sender: TObject);
    procedure redTrackBarChange(Sender: TObject);
  public
    { Public declarations }
    procedure ChangeColor;
  end;
 
procedure TForm1.ChangeColor;
begin
  Panel1.Color := RGB(redTrackBar.Position, greenTrackBar.Position, blueTrackBar.Position);
end;
 
procedure TForm1.blueTrackBarChange(Sender: TObject);
begin
  ChangeColor;
end;
 
procedure TForm1.greenTrackBarChange(Sender: TObject);
begin
  ChangeColor;
end;
 
procedure TForm1.redTrackBarChange(Sender: TObject);
begin
  ChangeColor;
end;

Взято с Delphi Knowledge Base: https://www.baltsoft.com/