Since its a percentage you would need to compare to 1 or multiply your value by 100.
= iif((fields!JulyRev.Value/Fields!JulyValue.Value) > 1, "SeaGreen","Black")
OR
= iif((fields!JulyRev.Value*100.0/Fields!JulyValue.Value) > 100, "SeaGreen","Black")