Alternative is to put the following code in your report (Report Properties -> Code)
Public Function BackColour(Byval line as Decimal) as String
If line = -1
return "Gainsboro"
End If
If line = -2
return "LightGrey"
End If
If line = -3
return "Silver"
End If
If line mod 2 = 0
return "#F08080"
Else
return "White"
End If
End Function
Now if your inner most details group, set the background color to the following :
=code.backcolour(runningvalue(Fields!sub_category_code.Value,countdistinct,nothing))
Hope this gives you the result you are after