[SuperZen]

javascript вариант,

<style>
  .invalid {
    color: red
  }
</style>

Есть вот такой select
<select name="CanFrame_Type" id="CanFrame_Type">
  <option value="Std" data-min-max="0-7FF">Std</option>
  <option value="Ext" data-min-max="800-FFFFFFFF">Ext</option>
</select>
Далее поле для ввода НЕХ
<input id="Can_ID" name="Can_ID" size="5" class="max1" />

<script>
  const select = document.getElementById('CanFrame_Type')
  const input = document.getElementById('Can_ID')
  const update = () => {
    var [min, max] = select.options[select.options.selectedIndex].dataset.minMax.split('-')
    var value = parseInt(input.value, 16)
    if (value.toString(16) === input.value.toLowerCase() && value >= parseInt(min, 16) && value <= parseInt(max, 16)) {
      input.classList.contains('invalid') && input.classList.remove('invalid')
    } else !isNaN(value) && !input.classList.contains('invalid') && input.classList.add('invalid')
  }
  select.addEventListener('change', update)
  input.addEventListener('input', update)
</script>