Pen Color

Think of the Graphics object as being like a sheet of paper,
some colored pens, and some drawing methods.
Use the following to change the color of the paper:

setBackground( Color.something )

Use the following to change to a different color pen:

gr.setColor( Color.something )

Several million colors are possible, but
for now, use the pre-defined colors:

Color.red, Color.green, Color.blue,  Color.white

These are static values from
the class Color.
(Look at your local documentation under
Color to find more choices.)

To draw a line on the Graphics object, use:

drawLine(int  x1, int  y1, int  x2, int  y2)

This draws a line from (x1, y1) to (x2, y2)
in the drawing area using the current pen color.
As usual,
the point (0,0) is the upper left corner of the drawing area.