What event are you putting this on?
Anyway, I do something like this:
Dim sText As String
If IsNull(Me.txtYourTextBox) or Me.txtYourTextBox = "" Then
sText = "Some value"
sText = Me.txtYourTextBox
Select Case sText
Case "Not Available"
Me.txtYourTextBox.ForeColor = vbRed
Case "Some other value"
Me.txtYourTextBox.ForeColor = vbGreen
Did that help any? You will need to copy and paste this to the On Current event, and the Before Insert and Before Update events on the form.