Conditional Expression

I have the following being applied as an expression:

=iif(sum(Fields!Percentile.Value) > 10, "Yellow", "Black")

I want my percents to show up yellow if they are above 10.00%, and black if they are under 10%, doesn't appear to be working right?one other question I will have with this is to find out if i can make the entire block yellow instead of just the text?