I just found this gem of a blog called Yellow Pages Power by Steve Yankee.  Almost all of his posts have a great little nugget of information about designing or writing ad copy for yellow pages.

In “Extras” Help Your Ad Outperform Others, I just learned this…

And here’s a little known fact for you: yellow pages aren’t really printed on yellow paper. It’s far more economical to print phone directories on white paper stock, and then use yellow ink to print the background color, and black ink to print the ads themselves. For years, having a yellow ad background was your only available choice but lately, most phone books have offered “white reverse” ad backgrounds, where your ad can feature a non-yellow background. This is another inexpensive trick to improving your ad readership.

I had actually wondered about that a few times.
Look Steve’s blog over.  It’s definitely worth the read.