Thank you for the question.
Richard's answer is valid for the visual text elements that our Telerik Presentation Framework provides - LightVisualElement and TextPrimitive
. Since these are our own elements, you can change their color depending on various user conditions.
However, this is not the case with RadTextBox. RadTextBox internally hosts the standard TextBox. This TextBox, however, does not allow for changing its ForeColor when it is disabled, hence you can't change the appearance of the text in RadTextBox when it is disabled.
If you have additional questions, feel free to contact us.
the Telerik team