When you make a WinForm TextBox ReadOnly, it aquires a distinctive look because it changes the background.  Users often want the appearance of the TextBox to stay the same, they just don't want it to be mutable.  Here's a snippet to make a TextBox ReadOnly and not change it's appearance.

Dim saved As Color = Me.TextBox1.BackColor

Me.TextBox1.ReadOnly = True

Me.TextBox1.BackColor = saved