The plugin doesn’t currently allow styling of individual icons via the color settings, sadly.
You could override the colors with CSS, though. For example, the following CSS added to your theme’s stylesheet or via a Custom CSS plugin should set the icon background colors of the most popular icons:
.simple-social-icons.widget ul li a,
.simple-social-icons.widget ul li a:hover {
color: #fff !important;
}
.simple-social-icons.widget ul li a:hover {
opacity: 0.8;
}
.simple-social-icons.widget ul li.social-facebook a {
background-color: #3d5a98 !important;
}
.simple-social-icons.widget ul li.social-twitter a {
background-color: #09b0ed !important;
}
.simple-social-icons.widget ul li.social-gplus a {
background-color: #dd4c39 !important;
}
.simple-social-icons.widget ul li.social-linkedin a {
background-color: #035a87 !important;
}
.simple-social-icons.widget ul li.social-rss a {
background-color: #f48b24 !important;
}
You’re welcome to add to and modify this if you wish.