Specials fields

Probably you’ll have to use javascript for this. If you are using a dropdown for your city field, you can alter the color of a specific dropdown option with some CSS:

option[value="MOLINA DE SEGURA"] {
    background-color: green;
}

But if you are using a text field you need to use javascript that looks at the value the user typed in and change the color if it matches a pattern. You’ll need to know some javascript and PHP to set that up.