:nth-child(2)

by Alex

HTML

<div class="b"></div>
<div class="b"></div>
<div class="b"></div>
<div class="b"></div>
<div class="b"></div>
<div class="b"></div>
<div class="b"></div>
<div class="b"></div>
<div class="b"></div>
<div class="b"></div>

CSS

.b {
    background: #f00;
    width: 20px;
    height: 20px;
    margin: 5px;
}
.b:nth-child(2) {
    background: #0f0;
}