Google’s AdSense product has offered a number of ways to design and customize ad units and styles over the years. Starting with strict HTML embeds of the colors and formats, then moving to a Google-hosted ad unit — allowing you to manage an unit on google.com/adsense.
Google is stepping up the game and allowing publishers to more effectively create ad units that will hopefully convert better — for both publishers and Google.
The ad unit customizer allows you to choose a contrast type, and then customize the colors based on the contrast.
WebdesignLedger.com has some further reading on The Principle of Contrast in Web Design.