div {
border-width: medium;
padding: 5px;
margin: 5px 10px;
border-style: solid;
border-width: thick;
}
.ex1 {border-color: red;}
.ex2 {border-color: green #f00;}
.ex3 {border-color: green #f00 blue;}
.ex4 {border-color: green #f00 blue #fe9503;}
<!DOCTYPE html>
<html lang="ko">
<head></head>
<body>
<div class="ex1">border-color: red;</div>
<div class="ex2">border-color: green #f00;</div>
<div class="ex3">border-color: green #f00 blue;</div>
<div class="ex4">border-color: green #f00 blue #fe9503;</div>
</body>
</html>