Цвет пикселя!

procedure TForm1.Timer1Timer(Sender: TObject);
 var
   i,j : integer;
begin
slider.Left := slider.Left + 1;
 i := Slider.Left-(line.Left)-1;
 With Line do
Begin
Canvas.Pen.Color:=clGreen;
   for j:= 1 to 10 do
 if  Canvas.Pixels[i,j]=clWhite then Canvas.Pixels[i,j]:=RGB((0), (255), (10));
end;
end;

procedure TForm1.Timer1Timer(Sender: TObject);
 var
   i,j : integer;
begin
slider.Left := slider.Left + 1;
 i := Slider.Left-(line.Left)-1;
 With Line do
Begin
   for j:= 1 to 10 do
 if  Canvas.Pixels[i,j]=clWhite then Canvas.Pixels[i,j]:=clGreen;
end;
end;

  for i := 0 to Image1.Height do
    for j := 0 to Image1.Width do
      if Image1.Canvas.Pixels[j, i] = clWhite then
        Image1.Canvas.Pixels[j, i] := clRed;

for i := 0 to Image1.Height - 1 do // !
    for j := 0 to Image1.Width - 1 do // !
...

With Line do
Canvas.Pen.Color:=clGreen;

Line.Canvas.Pen.Color:=clGreen

procedure TForm1.FormCreate(Sender: TObject);
begin
DoubleBuffered := true;
line.Picture.Bitmap.PixelFormat := pf24bit; // <---
end;