bootstrap4.css
.alert-secondary {
  color: #383d41;
  background-color: #e2e3e5;
  border-color: #d6d8db;
}
bootstrap4.css
.alert-secondary hr {
  border-top-color: #c8cbcf;
}
bootstrap4.css
.alert-secondary .alert-link {
  color: #202326;
}
■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>