Georgie0409,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
.main_container{
display: flex;
height: 8vmax;
width: 8vmax;
}
.main_container > .container{
height: 2vmax;
width: 2vmax;
border: 1px solid #0000CD;
}
</style>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.4/jquery.min.js"></script>
<script>
$(function() {
var divs = $('.container');
$('.main_container').on('click', '.container', function() {
divs.css({'background-color' : ''});
$(this).css({'background-color' : 'rgba(192,192,192, 0.25)'});
})
});
</script>
</head>
<body>
<div class='main_container'>
<div class='container'></div>
<div class='container'></div>
<div class='container'></div>
<div class='container'></div>
</div>
</body>
</html>