Here's the core code. The variables are declared at the top of the report module, the project variable is set in the load event. Feels like a kludge, but it works.
Code:
Private Sub Detail_Format(Cancel As Integer, FormatCount As Integer)
If strProject <> Me.Project Then
strProject = Me.Project
booChanged = Not booChanged
End If
If booChanged Then
Me.Detail.BackColor = RGB(204, 204, 204)
Me.Detail.AlternateBackColor = RGB(204, 204, 204)
Else
Me.Detail.BackColor = vbWhite
Me.Detail.AlternateBackColor = vbWhite
End If
End Sub