This is a pretty cool App, I love all the options you put in to the canvas for painting; though a way to control the alpha colors would be nice. A tool for zooming in would be pretty helpful as well.
Despite its features, user interactivity is not really the strong point in this app, as it takes too long for people to interact with one another and when they do it is very momentary and limited. This would be an interesting tool for when you're out of ideas and decide to go out scribbling something worthwhile, and maybe get at least one person to look at it. Using predefined words and such also cuts down the game by a little bit.
I suggest you look into other similar applications like "Doodle Or Die", which is entirely moved by user interaction and has little restrictions when it comes to what you can do with it. Otherwise, great work on this flash app.