Text box colour change

Hi there just wondering if its possible to make a text box in java script and then for that box 2 change colour when the user inputs something. For example if they enter 3 it will turn green but if they enter anything else then it will turn red.
Thanks

here's the cheap and nasty way to do it:
<input type="text" onkeyup="this.style.backgroundColor=this.value==3?'green':'red'" />