Change a CSS style when editing a post.

Ok I have been looking for a few hours now and I just cant seem to find this function anywhere. So I am hoping someone out there has a good solution.

I am building a custom theme for a friend and I want her to have the ability to set a specific “color” for each post she makes. (and go back and change it if needed).

The “color” she is changing basically changes the background color of a specific div element in my theme. So for each post she makes she gives it a custom style. I am trying to make things easy for her as she is not that technical to go and edit CSS files herself. Perhaps a plugin out there?