Don't know if you're still working on this but here's what I used:
rowColor = (count(preceding-sibling::myRepeatingSection) + 1) mod 2
Then, set the conditional formatting for the repeating section to color if rowColor <> 0
count( preceding-sibling::myRepeatingSection)+1 returns the rowNumber
x mod y returns the remainder of x/y. When y=2, odd numbers will return a remainder >0. Even numbers will return a 0.
You have to use a repeating section with the "header" outside of the section. Repeating tables don't work; I can only guess because the conditional formatting property applies to the entire control, a repeating table doesn't know what to do.
-smaus
***UPDATE: Tested and you can use a Repeating table but you cannot have headers or footers in it.