:focus CSS pseudo-class

label {
  display: block;
  margin-top: 1em;
}

input:focus {
  background-color: lightblue;
}

select:focus {
  background-color: ivory;
}

<form>
  <p>Which flavor would you like to order?</p>
  <label>Full Name: <input name="firstName" type="text"></label>
  <label>Flavor:
    <select name="flavor">
      <option>Cherry</option>
      <option>Green Tea</option>
      <option>Moose Tracks</option>
      <option>Mint Chip</option>
    </select>
  </label>
</form>

:focus {
  /* ... */
}

<div><input class="red-input" value="I'll be red when focused." /></div>
<div><input class="blue-input" value="I'll be blue when focused." /></div>

.red-input:focus {
  background: yellow;
  color: red;
}

.blue-input:focus {
  background: yellow;
  color: blue;
}