Since you are converting the
sender
event handler parameter to a button, and since that button will be the button that initiated the event, why don't you just use it directly instead of going round the houses with substring and related rubbish?
Public Sub Buttons_Click(ByVal sender As System.Object, ByVal e As System.EventArgs)
Dim btn As Button = CType(sender, Button)
btn.BackColor = Color.Red
End Sub