В итоге должно получится вот так.
<!DOCTYPE html>
<head>
<meta charset="UTF-8">
<title>Example 2</title>
<link rel="stylesheet" href="css/style.css">
</head>
<body>
<div id="block" class="flex-container center">
<div class="flex-item color"></div>
</div>
<div id="block" class="flex-container center">
<div class="flex-item color"></div>
<div class="flex-item color"></div>
<div class="flex-item color"></div>
</div>
<div id="block" class="flex-container center">
<div class="flex-item color"></div>
<div class="flex-item color"></div>
<div class="flex-item color"></div>
<div class="flex-item color"></div>
<div class="flex-item color"></div>
</div>
</body>
</html>