In Unit1.h    ~~
class TForm1 : public TForm
{
__published:        // IDE-managed Components
        TTrackBar *TrackBar1;
        TButton *Button1;
        TColorBox *ColorBox1;
        void __fastcall Button1Click(TObject *Sender);
private:        // User declarations
        void __fastcall TrackBarWndProc(TMessage&);  <--一樣增加這一行
         TWndMethod      OldTrackBarWndProc;          <--一樣增加這一行
public:                // User declarations
        __fastcall TForm1(TComponent* Owner);
};
//---------------------------------------------------------------------------
extern PACKAGE TForm1 *Form1;
~~

In Unit1.cpp    ~~
TForm1 *Form1;    TColor color=clSilver;     <--Tumb的default顏色    //---------------------------------------------------------------------------
__fastcall TForm1::TForm1(TComponent* Owner)
        : TForm(Owner)
{
 OldTrackBarWndProc    = TrackBar1->WindowProc;
 TrackBar1->WindowProc = TrackBarWndProc;
}
//---------------------------------------------------------------------------
void __fastcall TForm1::Button1Click(TObject *Sender)
{
 color=ColorBox1->Selected;
 SendMessage(TrackBar1->Handle,WM_PAINT,0,0);      <--依選擇改變顏色
}
//---------------------------------------------------------------------------
void __fastcall TForm1::TrackBarWndProc(TMessage& Message)
{
 OldTrackBarWndProc(Message);     if (Message.Msg==WM_PAINT) {      <--如果是繪、重繪事件時
     RECT rect;
     SendMessage(TrackBar1->Handle,TBM_GETTHUMBRECT,0,(LPARAM)&rect);  <--先取得Tumb的長方形         HDC dc=GetDC(TrackBar1->Handle);   取得TrackBar的DC，準備在上面劃自已想要Tumb的顏色
     TBrush *bsh=new TBrush;
     TPen *pen=new TPen;         bsh->Color=color;               <--brush屬性要設你好的顏色
     pen->Color=color;               <--pen屬性也要設你好的顏色         SelectObject(dc,bsh->Handle);   Select過brush和pen才真正能用
     SelectObject(dc,pen->Handle);         /*定義新的Tumb長方形，新長方形是為了做出立體、陰影的效果*/
     RECT new_rect;
     new_rect=rect;
     new_rect.bottom -= 5;
     new_rect.top     = 0.5;
     new_rect.left    = 1;
     new_rect.right  -= 1;         /*定義三角形、同樣加加減減的微調也是要做立體、陰影的效果*/
     POINT point[3];
     point[0].x=new_rect.left 1;
     point[0].y=new_rect.bottom;
     point[1].x=new_rect.right-2;
     point[1].y=new_rect.bottom;
     point[2].x=(new_rect.right new_rect.left)/2;
     point[2].y=rect.bottom-3;
    
     FillRect(dc,&new_rect,bsh->Handle);    <--開始劃長方形
      Polygon(dc,point,3);                   <--開始劃三角形
      
      DeleteObject(bsh);
     DeleteObject(pen);
     DeleteDC(dc);
 }
}
//---------------------------------------------------------------------------