You can consider the Delphi form as a canvas just like an artists canvas, and this is where you do your art work.
The Canvas has a Handle property.
And in Delphi we can use the Canvas property for drawing on forms and as you can see from the examples below you can do a lot with the Canvas.

Win32 uses the Graphics Device Interface (GDI) to help us (the programmer) to draw or paint images on the computer screen. 

Depending on what monitor you have some of the examples below may not work.

Make sure you take the time to look at Borlands demonstration program graphex.dpr you will find this in the 
Delphi\Demos\Doc\Graphex directory.
 

One of the advantages of using a TImage control is that refreshing and painting of the graphic image in the TImage is automatically handled by the VCL.
 

This is used to fill areas and this includes filling the interior of shapes.

The 3 brush propeties you can use are:

Style
Color
Bitmap

Examples:

Style:

Canvas.Brush.Style := bsBDiagonal;

Brush Styles = bsSolid, bsClear, bsBDiagonal, bsFDiagonal, bsCross, bsDiagCross, bsHorizontal, bsVertical.

Color:

Canvas.Brush.Color := clBlue;

Bitmap:

procedure TForm1.Button1Click(Sender: TObject);
var Bitmap : TBitmap;
begin
    Bitmap := TBitmap.Create;
    try
     Bitmap.LoadFromFile('e:\Images\Athena2.bmp');
     Canvas.Brush.Bitmap := Bitmap;
     Canvas.FillRect(Rect(20, 20, 200, 200));
    finally
      Canvas.Brush.Bitmap := Nil; {Set to nil}
      Bitmap.Free; {Free the bitmap, Delphi doesn't do this so we have to free this resource ourselves}
    end; 
end;

More examples are shown below.

Pen Mode:

pmWhite, 
pmBlack, 
pmNop (Invisible), 
pmNot, 
pmCopy, 
pmNotCopy (The inverse pen color), 
pmMergePenNot, 
pmMaskPenNot, 
pmMaskNotPen, 
pmMergeNotPen, 
pmMerge, 
pmNotMerge, 
pmMask, 
pmNotMask, 
pmXOR, 
pmNotXOR. 

Use the Delphi help files to learn more about these, look for pen.mode.
An easy way to find TPen.Mode is to paste or type the text into the Code Editor and then press the F1 key. 
Make sure the caret is inside the text.

Pen Color:

Canvas.Pen.Color := clBlue;

Pen Width:

Canvas.Pen.Width := PenWidth.Position;

Pen Style:

Canvas.Pen.Style := psSolid;

Pen Styles:
 

psClear	Transparent Line
psDash	Dashed Line
psDashDot	Line of Dashes & Dots
psDashDotDot	Line of 1 Dash & 2 Dots
psDot	Dotted Line
psSolid	Solid Line
psInsideFrame	A solid line and it can use a dithered color with a with greater than one pixel wide.

Note: Windows 95 doesn't support dotted or dashed lines styles for pens that are wider than one pixel and no matter what style you choose makes all larger pens solid.

More examples are shown below.

RGB = Red, Green, Blue.

Example:

RGB(0, 0, 0);

procedure TForm1.Button1Click(Sender: TObject); 
begin
    Form1.Color := RGB(100, 100, 100); 
end;

Each number should be from 0 to 255. 

By changing the values you can get different colors.

The upper left corner of the form is Canvas.Pixels(0, 0);
The bottom right corner is CanvasPixels(ClientWidth, ClientHeight);

Example:

This example will draw yellow stars (Dots) on a Form(Canvas):

Start a new project.
Change the colour(Color) of Form1 to clBlack.
Add Button1.
Add this code to the button’s OnClick event handler:

procedure TForm1.Button1Click(Sender: TObject);
var I : Integer;
begin
    I := 0;
    begin
        repeat
           I := I + 1;
          Canvas.Pixels[Random(ClientWidth), Random(ClientHeight)] := clYellow;
        until I = 10000;
   end;
end;

Run the program and click on Button1.

The Canvas.Font property is used to change the appearance of the text that is drawn on the canvas.

Using bold and underlined:

Canvas.Font.Style := [fsBold, fsUnderline];

Using a TFontDialog to change the forms font:

procedure TForm1.Button2Click(Sender: TObject);
begin
    Canvas.TextOut(10, 10, 'Here!');{Current forms font}
    if FontDialog1.Execute then
     Canvas.Font.Assign(FontDialog1.Font);
     Canvas.TextOut(100, 100, 'Here!');{New form font}
end;
 

Examples:

Canvas.Font.Name:

Canvas.Font.Name := 'Arial';

Canvas.Font.Color:

Canvas.Font.Color := clBlue;

Large Fonts:

This example was created like this:

procedure TForm1.Button1Click(Sender: TObject);
begin
    Canvas.Font.Height := Font.Size * Font.PixelsPerInch div 10;
    Canvas.Font.Name := 'Napa Heavy SF';
    Canvas.TextOut(100,  200, 'Text is here!');
end;
You can also change the font size with Font.Size

Adding text to a form:

procedure TForm1.Button1Click(Sender: TObject);
begin
    Canvas.TextOut(10,  30, 'Here!');
end;

Drawing a box with some text in it:

procedure TForm1.Button1Click(Sender: TObject);
var AreaBox : TRect;
begin
    AreaBox := Rect(100, 100, 200, 200);
    Canvas.TextRect(AreaBox,100, 100,'Inserted text');
end;

Drawing the same box as above, this time with a blue frame:

procedure TForm1.Button1Click(Sender: TObject);
var AreaBox : TRect;
begin
    AreaBox := Rect(100, 100, 200, 200);
    Canvas.TextRect(AreaBox,100,100,'Inserted text');
    Canvas.Brush.Color := clBlue;
    Canvas.FrameRect(AreaBox);
end;
 

Bitmap.Assign(Bitmap2);

StretchDraw(ARect, Image1.Picture.Bitmap);

You can use MoveTo to move the drawing position to somewhere else on the form like this:

Canvas.MoveTo(0, 0);

Have a look at the drawing example to see MoveTo in action.

LineTo is used to draw a line from its current position to a new position.

Here is an example:

Canvas.LineTo(10, 100);

You could do this:

Canvas.MoveTo(0, 0);      //Start in the top left corner of the form.
Canvas.LineTo(10, 10);   //Draw a line from 0, 0 to the new position at 10, 10.

Have a look at the drawing example to see LineTo in action.

GetNearestColor is a Windows function that can take a HDC and RGB color and return the nearest RGB color in the system palette.

CLR_INVALID is the return value if the function fails. Use GetLastError to get extended error information.

Example:

procedure TForm1.Button1Click(Sender: TObject);
var TheColor : TColorRef;
begin
  TheColor := GetNearestColor(Form1.Canvas.Handle, RGB(133, 53, 20));
  Panel1.Color := TColor(TheColor);
end;
.

To do this I added a TPanel (Panel1), and a TImage (Image1) to a form.

Next I loaded an image into Image1.

Next I clicked on Image1 to make sure it was selected and then I went to the Events page in the Object Inspector and double-clicked on the OnMouseDown event.

Next I added this code:

procedure TForm2.Image1MouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  Panel1.Color := Image1.Canvas.Pixels[X,Y];
end;

Run the example and click on an area in Image1. You should see the color appear im Panel1 now.
Click on different areas on the image.
.

Unit Graphics - Graphics Utilities

The ColorToRGB function is used to convert a TColor into a RGB representation of that color.

Example:

procedure TForm1.Button1Click(Sender: TObject);
var L : LongInt;
begin
  L := ColorToRGB(Form1.Color);
  Panel1.Color := L;
  Label1.Caption := ColorToString(L);
end;
.

procedure TForm1.Button1Click(Sender: TObject);
begin
    Canvas.Ellipse(100, 100, 160, 160);
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
    Canvas.Rectangle(100, 100, 200, 160);
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
    Canvas.Polygon([Point(0, 0), Point(50, 20),
    Point(150, 50), Point(240, 120)]);
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
    Canvas.Brush.Color := clBlue;
    Canvas.Polygon([Point(0, 0), Point(50, 20),
    Point(150, 50), Point(240, 120)]);
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
    Canvas.BrushStyle := bsSolid;
    Canvas.Brush.Color := clBlue;
    Canvas.FillRect(ClientRect);
end;

This could be used to clear the form, in other words rub-out the current content.

You can also use ClearCanvas.

procedure TForm1.Button1Click(Sender: TObject);
begin
   Canvas.RoundRect(0, 0, 60, 60, 40, 40);
end;

procedure TForm1.FormMouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
    if ssLeft in Shift then
    begin
        Canvas.Rectangle(X, Y, X + 10, Y + 10);
    end;
end;
Run the program and click the left mouse button while the mouse cursor is on the Form.

procedure TForm1.FormMouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
    if ssLeft in Shift then
     begin
         Canvas.Pen.Color := clBlue;
         Canvas.Rectangle(X, Y, X + 10, Y + 10);
         Sleep(200);
         Canvas.Rectangle(X, Y, X + 20, Y + 20);
         Sleep(200);
         Canvas.Rectangle(X, Y, X + 30, Y + 30);
         Sleep(200);
         Canvas.Rectangle(X, Y, X + 40, Y + 40);
   end;
end;
When you run this program click on the Form about 10 times as fast as you can, and all over the Form.

procedure TForm1.Button1Click(Sender: TObject);
var MyBitmap : TBitMap;
begin
    MyBitmap := TBitmap.Create;
    try
      with MyBitmap do begin
         LoadFromFile('D:\Delphi 3\Images\Splash\256color\handshak.bmp'); //Put your own .bmp file path here
        Form1.Canvas.Draw(10, 10, MyBitMap);
     end;
  finally
      MyBitmap.Free;
  end;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
    Form1.Canvas.Brush.Style := bsHorizontal;
    Canvas.Rectangle(10, 10, 60, 60);
    Form1.Canvas.Brush.Style := bsVertical;
    Canvas.Rectangle(70, 10, 120, 60);
    Form1.Canvas.Pen.Color := clBlue;
    Form1.Canvas.Brush.Style := bsCross;
    Canvas.Rectangle(130, 10, 180, 60);
    Form1.Canvas.Brush.Style := bsBDiagonal;
    Canvas.Rectangle(190, 10, 240, 60);
    Form1.Canvas.Brush.Color := clYellow;
    Form1.Canvas.Brush.Style := bsDiagCross;
    Canvas.Rectangle(250, 10, 300, 60);
end;

To do this I added a TPanel (Panel1), and a TImage (Image1) to a form.

Next I loaded an image into Image1.

Next I clicked on Image1 to make sure it was selected and then I went to the Events page in the Object Inspector and double-clicked on the OnMouseDown event.

Next I added this code:

procedure TForm2.Image1MouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  Panel1.Color := Image1.Canvas.Pixels[X,Y];
end;

Run the example and click on an area in Image1. You should see the color appear im Panel1 now.
Click on different areas on the image.
.