Bootstrap 4

HTML

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-alpha.2/css/bootstrap.min.css">
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-alpha.2/js/bootstrap.min.js"></script>
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootswatch/3.3.7/flatly/bootstrap.min.css">
<button class="btn btn-primary btn-block">Test</button>

CSS

.btn-primary {
  background-color: blue;
}

.btn-primary:hover {
  background-color: blue;
}

// WHERE IS THIS COLOR DEFINED WITHIN THE SCSS FILE OF BOOTSTRAP?
.btn-primary:not(:disabled):not(.disabled):active{
  
}