Detect Touch in irregular shape

hi,..
I am new to iPhone/iPad Programming
And i am developing demo application... So, i need help of all of you

Here i have described some points of my application.

1.) I have taken one Scroll View on the UIView and have put one ImageView on the scroll View.
2.) I have code to zoom only some parts of the image. And when user zoom any of those part, he/she can scroll the image as well.
3.) I want to fill color in some irrregular shape when touch is detected by the user.

    Eg. If there is image of face of one woman. I allow zooming of nose to lips of the image. Nw if i want to fill color in the lips then how to do that on the touch event?

Is it possibe what I am trying to do?
If yes then plz help me in these.

Thanks...