In the Color property of your text box, not in the Format property.
Please add your expression to the Color property of the text box, something like this
=IIF(ReportItems!Textbox15.Value < .05, "Red", "Blue")
hth
Tom in Sacramento - For better, quicker answers on T-SQL questions, click on the following...http://www.sqlservercentral.com/articles/Best+Practices/61537/