How to give color to the display of keyfigure based on condition using exception.

Dear Friends.

   I am trying to color "BAD3" in exception based on condition but my problem is in exception I can have only formula variable to compare the value, How to assign a value to formula variable in BEx Query designer.

What I am trying to do is :

   in Query designer :

   I have PO Quantity and Delivered Quantity. 

  if PO Qnantity > Delivered Quantity

    then Delivered Quantity field should be colored as "BAD3" in exception.

but here proble is in exception

  I have alert level , operator, and  value fields for Delivered Quantity keyfigure ( Under definition tab - Exception is defined on = Delivered Quantity ).

but for value field I dont have PO Quantity for that I have to supply one formula variable,

When I created a forumula  and did this way

FV_PO_QUANTITY = PO_QUANTITY formula editor throws errors. I dont understand How to assign a value of key figure to formula variable and use it in EXceptions.

Please help me How I can solve my problem

I will greatly appreciate your any help.

Thanking you

Regards

Naim