SOURCE

<p class="p_0">background-color：默认为 空</p>
<p class="p_1">background-color：red</p>
<p class="p_2">background-color：#ff0000</p>
<p class="p_3">background-color：rgb(255,255,0)</p>
<p class="p_4">background-color：rgba(255,0,0,.5)</p>
<p class="p_5">background-color：transparent 透明</p>

p{
    margin-top: 10px;
    padding: 10px;
    border: 10px solid rgba(10,10,10,.2);
}
.p_1{
    background-color: red;
}
.p_2{background-color: #ff0000;}
.p_3{
    background-color: rgb(255,255,0);
}
.p_4{
    background-color: rgba(255,0,0,0.5);
}
.p_5{
    background-color: transparent;
}