<h2>#c000</h2>
<div class="box box0"></div>
<h2>#000c</h2>
<div class="box box1"></div>
<h2>#000</h2>
<div class="box box2"></div>
<h2>#000000cc</h2>
<div class="box box3"></div>
.box{
width: 100px;
height: 100px;
}
.box0{
background: #cc000000;
}
.box1{
background: #000c;
}
.box2{
background: #000;
}
.box3{
background: #000000cc;
}