background-color
Syntax:

background-color: color | transparent


<style type="text/css">
.demo1 {
   background-color: #00FF00;
   width: 200px;
   border: 1px solid;
   height: 50px;
}
</style>
<div class="demo1"></div>
<style type="text/css">
.demo2 {
   background-color: red;
   width: 200px;
   border: 1px solid;
   height: 50px;
}
</style>
<div class="demo2"></div>
<style type="text/css">
.demo3 {
   background-color: rgb(0,0,255);
   width: 200px;
   border: 1px solid;
   height: 50px;
}
</style>
<div class="demo3"></div>
<style type="text/css">
.demo4 {
   background-color: transparent;
   width: 200px;
   border: 1px solid;
   height: 50px;
}
</style>
<div class="demo4"></div>