<div class="first"></div>
<div class="second"></div>
<div class="third"></div>
<div class="fourth"></div>

div{width:50px;height:50px;}
.first{background: rgba(0,255,0,1);}
.second{background: rgba(255,0,0,1);}
.third{background: rgba(0,255,0,0);}
.fourth{background: rgba(0,0,255,1);}