How do I change the color of the last menu item?

To change the color of the last menu item, just go to Appearance > Editor > style.css and look for the following style:  

#top-menu > li:last-child {
    padding-right: 0;
    background-color: #D6B840;
    font-size: 16px !important;
    margin: 0 0 0 0px !important;
    padding: 12px 24px 0 !important;
    border-radius: 2px !important;
}

Change the background-color value to whatever you’d like, save your changes and you’ll be all set!