Change icon on determined zone

Hello
I did a simple flash file to be able to users paint on draw. Now i want to change the icon only on certain zones but i don’t know how to do that…
can anyone help me?
I want the icon to be the pencil when it’s on the yellow area and i want it to be red again when it’s on the red area.

Here is my fla: http://www.endbox.com/temp/paint.fla

Thanks in advance