How to ersase pics in dialog

Let say I draw a line directly on a dialog.
How can I erase that line, so that it disapperas.
I know that I can get the background color, and then
paint over the line with it. But I think it is not
so god.

(What I do is drawing lines. After a line is drawn,
I will erase it and draw a new line.)

1. Is there some neutral pen that paint with the
background color?

2. Could it be that I must make some 'object' in the dialog
and paint on that, and than say to that object to
erase it self?

Please write some example how I can do these 2 alternative
solutions. Please tell me the best solution.

Thanks, Tomas