To set the color of the left border in JavaScript, use the borderLeftColor property. Set the color on this property that you want for the border.
Example
You can try to run the following code to learn how to set the left border color −
<!DOCTYPE html>
<html>
<head>
<style>
#box {
border: thick solid gray;
}
</style>
</head>
<body>
<div id="box">Demo Text</div>
<br><br>
<button type="button" onclick="display()">Change left border color</button>
<script>
function display() {
document.getElementById("box").style.borderLeftColor = "green";
}
</script>
</body>
</html>