border-bottom-color
Syntax:

border-bottom-color: color | transparent (In IE transparent becomes black)


<style type="text/css">
.demo1 {
   border: 4px solid;
   border-bottom-color: #00FF00;
   width: 200px;
   height: 50px;
}
</style>
<div class="demo1"></div>
<style type="text/css">
.demo2 {
   border: 4px solid;
   border-bottom-color: #0F0;
   width: 200px;
   height: 50px;
}
</style>
<div class="demo2"></div>
<style type="text/css">
.demo3a {
   border: 1px solid;
   background-color: #FF0000;
   width: 200px;
   height: 50px;
   align: center;
}
.demo3b {
   border: 10px solid;
   border-bottom-color: #0000FF;
   background-color: #00FF00;
   width: 100px;
   height: 25px;
}
</style>
<div class="demo3a"><div class="demo3b"></div></div>
<style type="text/css">
.demo4a {
   border: 1px solid;
   background-color: #FF0000;
   width: 200px;
   height: 50px;
   align: center;
}
.demo4b {
   border: 10px solid;
   border-bottom-color: transparent;
   background-color: #00FF00;
   width: 100px;
   height: 25px;
}
</style>
<div class="demo4a"><div class="demo4b"></div></div>