another richtextbox question

I have been familiarising myself with the richtextbox so that I can use it in the future. What I've been trying to do is have to text of the box updated by code, but with different colours but I can't seem to get it right.

Whenever I add text to the box, it is like the formatting set for the previous text is lost (while stepping through the code, the previous formatting the lost when the new text is added). Here is the code I am using:

Option Explicit
Private Const msg = "123"

Private Sub Command1_Click()
    rtb.Text = rtb.Text & msg
    rtb.SelStart = Len(rtb.Text) - Len(msg)
    rtb.SelLength = Len(msg)
    rtb.SelBold = True
    rtb.SelLength = 0
End Sub

Private Sub Command2_Click()
    rtb.Text = rtb.Text & msg
    rtb.SelStart = Len(rtb.Text) - Len(msg)
    rtb.SelLength = Len(msg)
    rtb.SelBold = False
    rtb.SelLength = 0
End Sub

Thanks,
Sunny