I couldn't get it to work for a continuous view (not saying it can't be done, just that I didn't get it), but did get it for Single record view. Code like this in the Form_Current event:
VB Code:
  1. Private Sub Form_Current()
  2.        If [txtFoo] = "B" Then
  3.             [txtFoo].BackColor = vbBlue
  4.       Else
  5.             [txtFoo].BackColor = vbWhite
  6.       End If
  7. End Sub
You'd have to alter it some for a combobox, but that's the idea. If you have to have a continuous form, I don't know without more research.