Nice Idea. Maybe easy to achieve by adding a rownummer property to the Template of a gallery. In this way you can also do three colors or four colors, etc.
The workaround that dhawalseth posted doesn't work when you have filtered the gallery because you're going to be missing some IDs, so instead of a nice list going 1,2,3,4,5,6 - you get something going 1,2,3,5,7.8 - which means you're not going to get alternate row colors.
Done it by using a sequential number in data and applied Mod function with 2 to determine odd/even and applied different Template fill property for odd and even by an if condition.
Galleries need an inherent Index property to represent rows, so that we do not have to modify Collections to include an Index. Then the gallery can have properties per Index - like font or color etc.