ComboBox with check if already selected or not

Good afternoon I need help to make a checkbox with the following function: if a checkbox value is already inserted it will be red, and if any value is not entered it will be green.

So this is my selectbox and what I need help is that when selecting an option this option will turn red and the rest will turn green, and then turn red when in this case it is for a barbershop, then

<select id="hora" name="hora"> 
    <option >Hora</option> 
    <option value="8:30" >8:30</option> 
    <option value="9:30">9:30</option> 
    <option value="10:30">10:30</option> 
    <option value="11:30">11:30</option> 
</select>