Tricky Conditional Formatting Calc Needed in Web Direct

[Rich S]

Howdy, howdy:

I have a phone number field and a button on a Web Direct form and what I want to happen is when a user enters the tenth digit of their phone number into the phone number field, the button's background color should change to green.

I thought a script trigger would be the ticket, but some of them aren't available in Web DIrect. What are my options?

 

Cheers,

Rich