I'm dabbling with YPN on a site, and I don't know how/if you can specify alternative ads. Is this possible?

Right now, the only thing it seems that I'm able to do is to display a substitute color. Is this all?

Also, I'm curious if its possible to detect (maybe using JavaScript) if the substitute color was displayed, any scripting leads you might suggest, like analysing the ad's iframe src onload?

Thanks