Create a CMS field in admin called: tb-back
On the page put the topbar inside a CSS Box stack
Set the css box class to: YourClassName
Add thefollowing CSS to the page/site:
.YourClassName .top-bar-wrapper{background: !important;}

If your menu has drop-downs you will also need the CSS for them:
.YourClassName ul.dropdown{background: !important;}

You can type the color into the admin field for tb-back and change it at will