Yesterday I added AdSense to my company blog as an experiment.
While you wait to get approved by AdSense, Google displays a "blank ad." But the ad wasn't blank, it was a pale yellow on the pinkish side.
I tried to change the default color for blank ads, but that didn't work.
The problem, I discovered, is that by default Wordpress assigns the ins tag to be yellow.
Because AdSense uses the ins tag to inject ads in the page, the blank AdSense box ends up that pinkish yellow color.
In Wordpress, this is the offending CSS (in style.css):
That's setting ins to be the yellowish background color.
To replace the pinkish yellow box with blank white, simply add this line to the style.css file of your child theme. (If you don't have a child theme, you should create one so that you can update your theme files without overriding your edits.)