An AdSense ad unit has 5 basic color elements. Having an understanding of these elements are useful when it comes to customizing an ad unit to match your blog's colors. The figure below shows a mapping of those elements to an actual ad unit.
The elements are Border, Background, Text, Title and Url. Border is the area where you see the "Ads by Google" part. This is in fact the background color of that rectangular area. The Background element is the background color of the area excluding the border. Each individual ad comprises of 3 elements, its title, the descriptive text and the ad URL. The Title, Text and Url elements correspond to these 3 elements of each ad respectively.
Whether you customize your ad unit using the Blogger Layout Editor or your AdSense account, these are the 5 color elements available for color customizations.
The actual color of each element is specified using a cryptic number similar to the #6699cc given for the Border element in the above figure. They are called the RGB color values. This article explains the RGB colors and a simple technique to find the exact RGB value for a given color is found here.