void gridview_CellMouseEnter(object sender, DataGridViewCellEventArgs e)
        {
            if (e.RowIndex == -1)
                DoWhatYouWant();
        }

------解决方案--------------------
你说的 索引超出范围问题这样解决：
private void dataGridView1_CellMouseEnter(object sender, DataGridViewCellEventArgs e)
       {
           dataGridView1.Rows[e.RowIndex -1].Cells[e.ColumnIndex].Style.BackColor = Color.Red;
       }