There are several reasons why you could get this error however, none of them is related to the RadColorPicker control. Could you please try to replace the RadColorPicker with a control from the ASP.NET AJAX Toolkit
and test if you get the same error?
In case you only get the error with the Telerik RadColorPicker control, open a new support thread and send us a simple running project that demonstrates this problem. Once we receive it we will do our best to help you.
the Telerik team