If you are using a wordpress script and using some theme, open your stylesheet and look for the background color codes... Do a trial and error by checking all colors that could math your ads... Use this tool: http://www.colorcombos.com/combotester.html if you found the background color, use that code to customize your adsense background... But of course there are some themes that uses images on background so we cannot match them... Simple and easy
Sure you can, just grab a screen shot, use your favorite editor, match the color code and your done. Just like my avatar.
Its the same tools with the one I posted above only the former have many options and you can pick multiple colors at the same time..