Try like this
for (int i = 0; i < dataGridView1.Rows.Count;i++ )
{
if ((string)dataGridView1.Rows[i].Cells[0].Value == "Yes")
{
dataGridView1.Rows[i].DefaultCellStyle.BackColor = Color.Green;
}
else
{
dataGridView1.Rows[i].DefaultCellStyle.BackColor = Color.Red;
}
}