Is there a callback or some mechanism that will allow me to modify a view once a field in a form is finished .... asumming that there are multiple fields in a form.
As a silly example, let's say the bacground of the filed is white by default ... but once the user goes on to the next field, then make the background of that field red if the number of characters in the field is even.
Well this would almost certainly have to be done in the browser using JavaScript so it's not really a Rails question.
Most Rails apps use either the Prototype JavaScript framework (since that's what the ajax helpers use), although the modern trend seems to be to use jQuery (and Rails 3 will make changing JavaScript frameworks much easier).
If you google for
prototype field validation and jquery field validation
you'll come up with serveral plugins for both frameworks.