In any type of complex matrix, when you want alternate cell colors, either row wise or column wise, the working solution is,
If you want a alternate color of cells coloumn wise then,
=IIF(RunningValue( Fields![ColumnGroupField].Value, countDistinct, "FakeParentGroup" ) MOD 2, "White", "LightGrey")
Thats all.
Same for the alternate color row wise, just you have to edit solution accordingly.
NOTE: Here, sometimes you need to set border of cells accordingly, usually it vanishes.
Also dont forget to delete value 1 in report that came into pic when you created fake parent group.