Masonry post colors

You can use a web dev tool like Firebug to inspect elements of your page, find the associated CSS class(es) and add some custom CSS (via a plug-in like Simple Custom CSS)

For example, on my page to add colored box around posts particular to their category, I added:

.wc-shortcodes-filter-tv
{border: 3px solid #F60;}

[Where ‘tv’ was one of the categories. The same process would work for changing the background of the post itself.]