You could do it by placing a body background colour style inside the head of the page and entering the CMS generated code inside it (removing the space before and after the code). Assuming the page is 'index.php', and you have called the edit region 'Page Colour', it would look like this:
echo WA_getCMSContent($database_localhost, $localhost, 'index.php' , 'Page Colour');
As PowerCMS by default seems to insert <p> </p> round anything you type in, you would have enter the code in 'Source' view (HTML Editor 'Source' button) and type the colour in there - otherwise the <p> tags will stop it working.
You would enter either 'white' 'black' 'gray' 'red' 'blue' etc, or a hexidecimal number (including hash) i.e. #FFFFFF #000000 #999999 etc. You would have to trust that your clients aren't going to type in, "Reddish-brown, with a hint of peach".
I'm not sure I am man enough to let a client loose with page background colours on my designs though!
Perhaps I'm over-complicating it though. There must be away to add it to the HTML editor stylesheet (except you can't select the 'body' tag).