Princy, thank you for your assistance.
Patxi, as it is pointed out in the forum link that Princy provided, you can change the BackColor of RadTextBox control by changing the BackColor of its subelements RadTextBoxItem and FillPrimitive:
|this.radTextBox1.TextBoxElement.TextBoxItem.BackColor = Color.Red;
|((FillPrimitive)this.radTextBox1.TextBoxElement.Children).BackColor = Color.Red;
If you have additional questions, feel free to contact me.
All the best,
the Telerik team