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