dgv.RowsDefaultCellStyle.BackColor = Color.White;
Technically that should already be set from when you added the grid view, but I do it just to make sure. So now that the default is set, we can specify the alternate colour...
dgv.AlternatingRowsDefaultCellStyle.BackColor = Color.Gray;
If you don't want to use the standard set colours, you can always specify your own...
dgv.AlternatingRowsDefaultCellStyle.BackColor = Color.FromArgb(240, 240, 240);
