bootstrap4.css
.text-primary {
  color: #007bff !important;
}
bootstrap4.css
a.text-primary:hover, a.text-primary:focus {
  color: #0056b3 !important;
}
■色Color:bg-primary:ユーティリティ:文字に色をつける

<div class="text-center">
  <p><a href="#" class="text-primary">text-primary</a></p>
  <p><a href="#" class="text-secondary">text-secondary</a></p>
  <p><a href="#" class="text-success">text-success</a></p>
  <p><a href="#" class="text-danger">text-danger</a></p>
  <p><a href="#" class="text-warning">text-warning</a></p>
  <p><a href="#" class="text-info">text-info</a></p>
  <p><a href="#" class="text-light bg-dark">text-light</a></p>
  <p><a href="#" class="text-dark">text-dark</a></p>
  <p><a href="#" class="text-body">text-body</a></p>
  <p><a href="#" class="text-muted">text-muted</a></p>
  <p><a href="#" class="text-white bg-dark">text-white</a></p>
  <p><a href="#" class="text-black-50">text-black-50</a></p>
  <p><a href="#" class="text-white-50 bg-dark">text-white-50</a></p>
</div>