Your code looks OK and the decorated textbox in such scenario should have coloured borders (see attached screenshot). If this does not happen on your side, please open a support ticket and send us a sample project where the problem could be reproduced.
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items.