Unless you are using a palette, Windows will attempt to convert pen and text colors to the nearest pure color available. You can determine the nearest pure color, a given color will map to, by calling the Windows API function GetNearestColor(). If you wish to use dithered colors for a pen, you can also set the pen style to the psInsideFrame style. When using this style, the pen draws lines within the frame of closed shapes that specify a bounding rectangle instead of centering the line over the shape. For the psInsideFrame style to have an effect, the pen width must be larger than pixel. Example:

procedure TForm .Button Click(Sender: TObject);
var NearestColor : TColorRef;
begin
  Form .Canvas.Brush.Color := RGB( 43, 53, 22);
  Form .Canvas.Pen.Color := RGB( 43, 53, 22);
  Form .Canvas.Pen.Width :=  0;
  Form .Canvas.Ellipse( 0,  0,  00,  00);
  Form .Canvas.Pen.Style := psInsideFrame;
  Form .Canvas.Ellipse( 0,   0,  00, 200);
  Memo .Lines.Add(
     'The nearest displayable color to RGB( 43, 53, 22) is:');
  NearestColor :=
    GetNearestColor(Form .Canvas.Handle,
                    RGB( 43, 53, 22));
  Memo .Lines.Add('Red := ' +
                  IntToStr(GetRValue(NearestColor)));
  Memo .Lines.Add('Red := ' +
                  IntToStr(GetGValue(NearestColor)));
  Memo .Lines.Add('Blue := ' +
                  IntToStr(GetBValue(NearestColor)));
end;