I can successfully change the border colour of the placeholder, but when I try change the background colour, nothing happens.
This jsfiddle code best explains it: http://jsfiddle.net/EUVrK/1/
According to the jQuery UI Sortable Documentation you can provide a css class for the placeholder. You should be able to specify the background in this class.
Using the placeholder and the forcePlaceholderSize property, it works like a charm. I've updated the jsfiddle link
Hope that helps.