According to MSDN you do the following:

Creating a Green-Bar Report To apply a green-bar effect (alternating colors every other row) to a table in a report, use the following expression in the BackgroundColor property of each text box in the detail row:

=iif(RowNumber(Nothing) Mod 2, "PaleGreen", "White")

To alternate colors by row groupings:

=iif(RunningValue({Expression Being Grouped On}, CountDistinct, nothing) Mod 2, "WhiteSmoke", "White")