Expression for Font Color

I need help with an expression for Font color.This is the current expression=IIF(SUM(Fields!Hours.Value) > Fields!Max_Hours.Value, "Red", IIF(SUM(Fields!Hours.Value) < Fields!Max_Hours.Value, "Green","Orange"))But sometimes the SUM(Fields!Hours.Value ) is NULL.I need to tweak the expression so that if its NULL then it treats it as zero and gives the color as Green.Currently its showing Orange.