:in-range

label {
  display: block;
  margin-top: 1em;
}
input:in-range {
  background-color: palegreen;
}

<form>
  <label for="amount">How many tickets? (You can buy 2-6 tickets)</label>
  <input id="amount" name="amount" type="number" min="2" max="6" value="4" />
  <label for="dep">Departure Date: (Whole year 2022 is acceptable)</label>
  <input
    id="dep"
    name="dep"
    type="date"
    min="2022-01-01"
    max="2022-12-31"
    value="2025-05-05" />
  <label for="ret">Return Date: (Whole year 2022 is acceptable)</label>
  <input id="ret" name="ret" type="date" min="2022-01-01" max="2022-12-31" />
</form>

<form action="" id="form1">
  <ul>
    1 から 10 の間の値が有効です。
    <li>
      <input
        id="value1"
        name="value1"
        type="number"
        placeholder="1 to 10"
        min="1"
        max="10"
        value="12"
        required />
      <label for="value1">あなたの値は</label>
    </li>
  </ul>
</form>

li {
  list-style: none;
  margin-bottom: 1em;
}
input {
  border: 1px solid black;
}
input:in-range {
  background-color: rgba(0, 255, 0, 0.25);
}
input:out-of-range {
  background-color: rgba(255, 0, 0, 0.25);
  border: 2px solid red;
}
input:in-range + label::after {
  content: "範囲内です。";
}
input:out-of-range + label::after {
  content: "範囲外です！";
}