How to set exception based on some amount value in % total field or Highlight anything >10% by another colour in SSRS 2008R2?

Right click the total textbox and set as like below.

Next go to backgroundcolor property of that textbox.

Set Expression as like below.

=iif( iif(iif(isnothing(Sum(Fields!Amount.Value,"Year")),0,Sum(Fields!AMOUNT.Value)/Sum(Fields!AMOUNT.Value,"Year"))=0,nothing,Sum(Fields!AMOUNT.Value)/Sum(Fields!AMOUNT.Value,"Year"))>0.1 ,"Green","White")

Thats it.