Need help in creating Color Combination logic for Matrix report

Hi Everyone,

I have created a matrix report having  "Probability"  in Row and "Impact" in Column  and "Risk ID" in Data Part .

                Now the problem which I am facing is I need to fill matrix with different color value based on combination of Probability and Impact 

I am able to write the expression in "Fill" area of "Risk_ID" text box but the problem is only those text box gets colored which are having Risk-Id value in it like shown below.I want all text box to be colored even if no Risk_Id is there in any text box.

If anyone  has any idea ,please let me know in writing the correct logic so that all text box gets colored even if there is no Risk _Id in it.

Any help will be greatly appreciated

Thanks Anuj