Alternate Fill Color

I would like to be able to alternate the fill color between white and gray as the data changes in COLUMN1.

The data in this column can be duplicated

For example

COLUMN1  COLUMN2

A 1 >>this should white
A 2 >>white
A 3 >>white
B 1 >> gray
C 1 >> white
C 2 >> white
D 1 >> gray
E 1 >> white
E 2 >> white
E 3 >> white
E 4 >> white
F 1 >> gray
F 2 >> gray

this is the detail of a tablix control

using ssrs 2008

thanks