The target is the size of standard 8.5"x11" paper, so you could print out anything and try it. In our tests, the app is better at detecting the laser when it is black lines on white paper. The scoring is calculated based upon a circular target, but you can change the size of the scoring area in the app.
If you need to re-print the target we shipped, here it is (right click and save to your computer):
A darker center is less reflective of the laser and may cause the app to miss detecting the shot. It will work, but it is most reliable when used in a darker lighting situation. You can print it out and try it in your iTarget sled. Right Click and "Save As"
An iTarget User created this one which has a color center and works very well for him.