Do you have the artwork as separate layers? If not, you'll want to break out the parts you want to write on, then cover them with paint strokes, animate the paint strokes with the write-on behavior, and use those paint strokes as an image mask applied to your yellow lines.
Freelance Producer/Editor/Motion Graphics Artist
Apple-certified Master Trainer
Author, Motion 4 from Peachpit Press http://www.applemotion.net
So great. Thank you. How do I use the paint strokes as an image mask. I managed to draw the tree (It drew white) I managed to write on the paint strokes, but what I can't yet figure out is how to make the draw on the yellow image behind it.