Use the Alternating Row Color code function and call it from the BackgroundColor property of the tablix row column.
Here is the Function that needs to be placed in the Code tab of the Report Properties window:
Function AlternateColor(ByVal OddColor As String, _
ByVal EvenColor As String, ByVal Toggle As Boolean) As String
If Toggle Then bOddRow = Not bOddRow
If bOddRow Then
Return OddColor
Else
Return EvenColor
End If
End Function
Here is the expression for the BackgroundColor property for column 1
=Code.AlternateColor("LightGrey", "White", True)
Here is the expression for the BackgroundColor property for column 2 and on
=Code.AlternateColor("LightGrey", "White", False)