Table: =iif(RowNumber(Nothing) Mod 2,"Green","White")
Matrix: =iif(RowNumber("Matrix_RowGroup") Mod 2,"Green","White")
The above is by far the most popular method of accomplishing the task, it is widely known and very simple. However it has one major downfall, when the matrix doesn't receive any data for a specific cell it must generate a blank cell, when that happens the RowNumber count does not get auto incremented which can really throw the coloring off.
Chris Hays came up with a great workaround hack to this issue. You can create a static grouping at the lowest level and use RunningValue to calculate an alternating value for the group header cell, then hide the cell and point the data cells BackgroundColor to the value of the group header cell.
=iif(RunningValue(Fields!Country.Value,CountDistinct,Nothing) Mod 2, "AliceBlue", "White")
I would highly recommend visiting Chris's blog where he gives step by step instructions on how to accomplish the task.