I fixed it---just re-sequenced some of my steps. Here it is, also cleaned up a bit

Public Sub rtbAppendText(NuText As String, Color As Long)
Dim oldLen As Long
On Error Resume Next
With rtbCmdLog 'this is the name of the rich tecxt box being used
oldLen = Len(.Text)
.SelStart = oldLen
.SelColor = Color
.SelText = .SelText & NuText
End With
End Sub