bootstrap4.css
.text-dark {
  color: #343a40 !important;
}
bootstrap4.css
a.text-dark:hover, a.text-dark:focus {
  color: #121416 !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>
■色Color:bg-primary:ユーティリティ:背景に色を付ける

<div class="text-center">
  <p><a href="#" class="bg-primary text-white d-block py-2">bg-primary</a></p>
  <p><a href="#" class="bg-secondary text-white d-block py-2">bg-secondary</a></p>
  <p><a href="#" class="bg-success text-white d-block py-2">bg-success</a></p>
  <p><a href="#" class="bg-danger text-white d-block py-2">bg-danger</a></p>
  <p><a href="#" class="bg-warning text-dark d-block py-2">bg-warning</a></p>
  <p><a href="#" class="bg-info text-white d-block py-2">bg-info</a></p>
  <p><a href="#" class="bg-light text-dark d-block py-2">bg-light</a></p>
  <p><a href="#" class="bg-dark text-white d-block py-2">bg-dark</a></p>
  <p><a href="#" class="bg-white text-dark d-block py-2">bg-white</a></p>
  <p><a href="#" class="bg-transparent d-block py-2">bg-transparent</a></p>
</div>