Свойство border-bottom-color

border-bottom-color: цвет;

<!DOCTYPE html>
<html lang='ru'>
<head>
<meta charset='UTF-8'>
<meta name='viewport' content='width=device-width, initial-scale=1.0'>
<title>Document</title>
</head>
<body>
<style>
#border {
width: 200px;
height: 100px;
border: 3px solid rgb(124, 75, 207);
border-bottom: 5px dashed;
border-bottom-color: orange;
}
</style>
<div id='border'></div>
</body>
</html>