Complex server-side rules with QuickForm

Hi All, I am wanting to do "complex" server-side JS validation. I put "complex" in quotes because complex is of course in the eye of the beholder. What I would like to do is actually fairly simple in theory. We have a drop down box, and a text box. We ask the user for their favorite color, and the options in the drop down box are red, green, blue, or other. If they choose red, green, or blue, then the text box is grayed out (disabled='disabled' attribute is set for the <input> tag). If they choose other, then the text box becomes available for them to input their favorite color in. Is there a way to do this with QuickForm without resorting to hand-coding all the JS? I have a vague sense that I once saw a tutorial on how to do this, but I've been googling for it and not been able to turn anything up. I would be very grateful for any tips! Sincerely, -Josh