.a {
width: 500px;
margin: 0 auto 1em;
background: #ffffff;
text-align: center;
border-style: solid;
border-color: #ff0000;
border-width: 5px;
}

.b {
width: 500px;
margin: 0 auto 1em;
background: #ffffff;
text-align: center;
border-style: solid;
border-color: #ff0000 #0000ff;
border-width: 5px;
}

.c {
width: 500px;
margin: 0 auto 1em;
background: #ffffff;
text-align: center;
border-style: solid;
border-color: #ff0000 #0000ff #008000;
border-width: 5px;
}

.d {
width: 500px;
margin: 0 auto 1em;
background: #ffffff;
text-align: center;
border-style: solid;
border-color: #ff0000 #0000ff #008000 #ffcc00;
border-width: 5px;
}