The demo that is linked
in the other thread is still live and is still a valid approach to do this, so you can use it as base for your development.
On the error with long text - I advise that you add a breakpoint in your code and step through it so you can see the actual server error, as what you receive is the MS AJAX wrapper for an error during a partial postback. It is also possible that you have exceeded the maximum request length allowed by your web.config (see maxRequestLength in httpRuntime).
On the difference between RadEditor and RadTextBox - RadEditor is a rich text editor that outputs HTML, so a colored text would be a combination of a span element (or some other container) and some CSS, the color property. This cannot be displayed in a RadTextBox, as it only takes a regular string and not HTML markup. If you want to display content taken from RadEditor, or generally some HTML you can use a simple wrapper like an ASP panel, Label or Literal. If you do not need the editing functionality of the RadEditor you needn't add it to the content, it will result in loading more HTML, more CSS and more scripts than needed.
All the best,
the Telerik team
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