Graphics help

I`m working on a app that you can draw lines…like circles, rectangles…
my problem is when a user draw a circle for example…i wanna be able to paint (change color) inside of this circle.

Ive tried working on Shapes with cacheAsBitmap = true and Bitmaps with BitmapData but it doesnt seens to work.

Any idea ?? i`m lost…rs

Thx