Работа монитором. Три цвета.

void __fastcall TForm1::Button1Click(TObject *Sender)
{
TCanvas* canvas=new TCanvas;
canvas->Handle=GetDC(0);
canvas->Pen->Width=0;
 
canvas->Brush->Color=clRed;
canvas->Rectangle(0,0,Screen->Width*1/3,Screen->Height);
 
canvas->Brush->Color=clGreen;
canvas->Rectangle(Screen->Width/3,0,Screen->Width*2/3,Screen->Height);
 
canvas->Brush->Color=clBlue;
canvas->Rectangle(Screen->Width*2/3,0,Screen->Width*3/3,Screen->Height);
 
delete canvas;
}
//---------------------------------------------------------------------------

__fastcall TForm1::TForm1(TComponent* Owner)
    : TForm(Owner)
{
  FormStyle = fsStayOnTop;
  SetBounds(0,0,Screen->Width,Screen->Height);
  BorderStyle = bsNone;
  WindowState = wsMaximized;
}

void __fastcall TForm1::FormPaint(TObject *Sender)
{
  Canvas->Brush->Color = clRed;
  Canvas->Rectangle(0,0,Width/3,Height);
  Canvas->Brush->Color = clLime;
  Canvas->Rectangle(Width/3,0,2*Width/3,Height);
  Canvas->Brush->Color = clBlue;
  Canvas->Rectangle(2*Width/3,0,Width,Height);
}

void __fastcall TForm1::FormMouseUp(TObject *Sender, TMouseButton Button, TShiftState Shift, int X, int Y)
{
  if (Button == mbRight) {
    Close();
  }
}