Using Report Builder 3.0, here is a way to alternate the background color of rows in your report. See first image.
When you have your report open in design mode, click on the left hand side of your row. See second image. This should highlight the row. In the Property Pane, go to the Fill section. Modify the BackgroundColor field to have the following expression:
=iif(RowNumber(Nothing) Mod 2, “Transparent”, “Gainsboro”)
Save report and run again to view change.