change graph color

[#1]

I Have a drill down graph where department => section => locations

each section have a sample size named (Size)

each location have a sample size named (sample size)

so when i select department the graph shows the sections in it . How can i change the graph color if the section doesnt exceed the Size???

I want same thing to be done when section is selected so the locations graph will also change colors

note that for locations color i used  the following expression:

if(sum(Count)-[Sample Size] >=0 ,RGB(113,48,161),RGB(193,80,77))

But this wont work if only section is selected