Unfortunately it is not possible to achieve this with the ASP RadChart. This is because of the fact that the RadChart renders the line series using a single color which comes from Appearance property of the series and does not depend on the value.
Excuse us for the inconvenience.
the Telerik team
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 their blog feed