Assuming that the field is called Variance, then add the following code to the OnFormat event of the Details section (assuming that the field is in this section)
Dim R As Report: Set R = Me
If R!Variance < 0 Then
R!Variance.FontBold = True 'bold
R!Variance.ForeColor = 255 'red
Else
R!Variance.FontBold = False 'not bold
R!Variance.ForeColor = 0 'black
End If
Hope that this helps,
Cheers,
Steve