[voraa]

<input id=weight type="text" name="weight" value="" />
<input id=price type="text" name="price" value="" />
  <select id=sel name="colour" required>
    <option hidden value="">Выберите цвет</option>
    <option value="20">Красный</option>
    <option value="70">Бирюзовый</option>
    <option value="40">Фиолетовый</option>
 </select>
 
<script>
  document.getElementById('sel').addEventListener('change',
    e => document.getElementById('price').value = e.target.value 
  )
</script>