The easiest way of doing this is by creating a new textbox in the detail section of your report, call it something like LineCount.
Then follow these steps in the Properties of the textbox:

Set visible to no

Set control sourceto =1

Set running sum to either Over Group or Over All

Select all the items in the row you want either grey or white

Go to Format > Conditional Formatting

Set Condition 1 to "Expression is and type [LineCount] mod 2 = 1
Then set the fill colour to your choice but make sure your default formatting is set

This page was originally ported from the UtterAccess Forums. It is based heavily or in part on a post by Steven Buick.
Alternating Colours on Rows in Reports