I have prepared a sample project for you with extended RadTextBox with assigned event handler for the client-side "OnValueChanged" event. You could add an RadAjaxPanel in the page and on the "OnValueChange" event to fire AJAX request and handle it on the server (get the text boxes values and save them).
Additionally, please note that "OnValueChanged" is firing right after the "OnValueChanging" and I am not sure if it will be appropriate for changing the background color. Instead, I have set the "cssClass" property and to the CSS class I have set the "background" of the input element when on focus.
Hope that helps.
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to the blog feed