How to change the color of some text in textbox - Silverlight 3?

I have a usual TextBox. I know in order to change the color of some text I can use Run tag. But when I try to use it imperatively instead declaratively (because I need to do that dynamically) I have the text in TextBox like

AAA<Run...>Text to be changed in color</Run>

and sure nothing in c开发者_高级运维olor changes. How can I change the color of some text in some program way? Thanks

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I'm fairly sure you have to use a RichTextBox for that, TextBox only works with plaintext.