In the Google AdSense Help Center, the article What color palettes are the most successful? recommends to blend your ads, it means that you should make the background (google_color_bg) and borders (google_color_border) of your ads the same color as the background of your page where the ad is placed. Then, In addition to that, I would also recommend that you make the title (google_color_link) and the URL (google_color_url) of your ads the same color as the hyperlink of your page where the ads are placed. And finally, I would apply the same rule for the color of the text (google_color_text).