A fill tool for a paint app?

i was trying to get my head round creating a fill tool for a paint program im creating in flash.
I was thinking for example if a user was to draw a circle, when the fill tool was used within that circle it could use a hit test to determin the edge of the circle, i then realised that the hit test methord uses movieclips, this being a bad thing as the shapes to be filled are being drawn in real time by the user:(

How would i write actionscript that acted as a fill tool? I know there is a fill command in the dictionary but i only know how to use it when drawing shapes in actionscript.

i have included the .fla file, its flash8 as2
Thanks vic.