In the Color property of the textbox that displays the value that should be red when it is below a particular threshhold, put a statement similar to the one below. Note that in this example the value in question is contained in a field called Field1 and the threshhold is 100.
=IIF(Fields!Field1.Value < 100, "Red", "Black")
For an even more dramatic effect you can use this same type of statement to alternate the BackgroundColor with Red and Transparent, and then alternate the Color with White and Black.