Alternating background color in tablix problem

Hi,

I have a table which is based on an ordered dataset. I want the background color to alternate for rows. I set the BackgroundColor==IIf((RowNumber("Tabla") MOD 2) , "Gainsboro", "White") where "Tabla" is the name of the tablix.

It seemed that the rows are colored randomly. Then I found out that SSRS defined the row number before applying the order by clause of the dataset.

Could anyone suggest what and how to set?

Thanks.