border-color
Syntax:

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


<style type="text/css">
.demo1 {
   border: 4px solid;
   border-color: #00FF00;
   width: 200px;
   height: 50px;
}
</style>
<div class="demo1"></div>
<style type="text/css">
.demo2 {
   border: 4px solid;
   border-color: #0F0;
   width: 200px;
   height: 50px;
}
</style>
<div class="demo2"></div>
<style type="text/css">
.demo3 {
   border: 4px solid;
   border-color: #FF0000 #00FF00;
   width: 200px;
   height: 50px;
}
</style>
<div class="demo3"></div>
<style type="text/css">
.demo4 {
   border: 4px solid;
   border-color: #FF0000 #00FF00 #0000FF;
   width: 200px;
   height: 50px;
}
</style>
<div class="demo4"></div>
<style type="text/css">
.demo5 {
   border: 4px solid;
   border-color: #FF0000 #00FF00 #0000FF #000000;
   width: 200px;
   height: 50px;
}
</style>
<div class="demo5"></div>
<style type="text/css">
.demo6a {
   border: 1px solid;
   background-color: #FF0000;
   width: 200px;
   height: 50px;
   align: center;
}
.demo6b {
   border: 10px solid;
   border-color: #0000FF;
   background-color: #00FF00;
   width: 100px;
   height: 25px;
}
</style>
<div class="demo6a"><div class="demo6b"></div></div>
<style type="text/css">
.demo7a {
   border: 1px solid;
   background-color: #FF0000;
   width: 200px;
   height: 50px;
   align: center;
}
.demo7b {
   border: 10px solid;
   border-color: transparent;
   background-color: #00FF00;
   width: 100px;
   height: 25px;
}
</style>
<div class="demo7a"><div class="demo7b"></div></div>