Hey you will need this, put it in your event :
Assume you have a combo_box : comboBox1
and a datagridview : dataGridView1
for (int i = 0; i < dataGridView1.Columns.Count; i++)
{
for (int j = 0; j < dataGridView1.Rows.Count; j++)
{
if (dataGridView1 [i, j].Value.ToString()==comboBox1.Text)
{
dataGridView1[i, j].Style.BackColor =Color.Yellow;
}
}
}