bootstrap4.css
.alert-danger {
  color: #721c24;
  background-color: #f8d7da;
  border-color: #f5c6cb;
}
bootstrap4.css
.alert-danger hr {
  border-top-color: #f1b0b7;
}
bootstrap4.css
.alert-danger .alert-link {
  color: #491217;
}
■アラート:alert:コンポーネント:アラートの色

  <div class="alert alert-primary" role="alert">
    alert-primary
  </div>
  <div class="alert alert-secondary" role="alert">
    alert-secondary
  </div>
  <div class="alert alert-success" role="alert">
    alert-success
  </div>
  <div class="alert alert-danger" role="alert">
    alert-danger
  </div>
  <div class="alert alert-warning" role="alert">
    alert-warning
  </div>
  <div class="alert alert-info" role="alert">
    alert-info
  </div>
  <div class="alert alert-light" role="alert">
    alert-light
  </div>
  <div class="alert alert-dark" role="alert">
    alert-dark
  </div>