Unfrotunately there is no way to iterate over all grid rows since the rows are virtualized and only a small sub set is available at any time. But there are several other approaches that you can try.
You could easily create a GridViewRow style that manipulates the background under a specific condition:
This XAML will change the background to LightGreen only when the Size property of a data item is equal to 100.
If you need to make more intricate calculations to determine which row should have its background changed you could use the RowLoaded event which is fire everytime a row comes into view.