You first need to find the name of your block. Then you need to add custom code to your page.
How to find your block ID
I use Firebug - once you have Firebug open, go to your site, control click on the block you want to locate and scroll down to "inspect element in Firebug".
Sometimes this search it is not exact in Firebug. It will find the text for instance but not the block. But if you scroll up the code, you'll get to something like:
Now copy that block ID somewhere (I use Notepad, something that won't add extra formatting. don't use Word for instance).
How to add custom code to color your block
Now go to manage site on that same page and up top click on Page Settings. Click on Advance and insert this code:
background-color: #000000; *this is where you put the 6 digit color you want*
padding: 10px; }
I also add some padding because once you have a colored box, you may not want the text going right up to the edge.
Note: do not include my note "this is where you put the 6 digit color you want" in your code.