Bu şekilde de çalışmadı hocam kodum aşağıdaki gibi.

  private void dataGridView1_CellContentClick_1(object sender, DataGridViewCellEventArgs e)
        {
            foreach (DataGridViewRow row in dataGridView1.Rows)
            {

                string RowType = row.Cells[1].Value.ToString();

                if (RowType == "toptanci")
                {

                row.Cells[1].DefaultCellStyle.BackColor = System.Drawing.Color.Red;
                }

                else if (RowType == "perakendeci")
                {

                   row.Cells[1].DefaultCellStyle.BackColor = System.Drawing.Color.Red;

                }
            }
        }

Formun tamamı ise su sekilde

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using MySql.Data.MySqlClient;
using MySql.Data;
namespace Ys_Bilişim
{
    public partial class Form2 : Form
    {

        public Form1 frm1;
        public Form3 frm3;


        public Form2()
        {
            InitializeComponent();

            frm3 = new Form3();
            frm3.frm2 = this;



        }
        MySqlConnection baglan = new MySqlConnection("Data Source=85.25.139.59;Database=efektasa_kredi;User ID=efektasa_kredi;Password=*;");
       

        public Form1 Mdiparent { get; set; }
        
        
        
        DataTable tablo = new DataTable();
        MySqlDataAdapter baglayici = new MySqlDataAdapter();
        MySqlCommand komut = new MySqlCommand();

        private void Form2_Load(object sender, EventArgs e)
        {
            string sqlsorgu = "Select * from kredi order by tarih DESC";
            DataTable tablo = new DataTable();
            komut.CommandText = sqlsorgu;
            komut.Connection = baglan;
            baglayici.SelectCommand = komut;
            baglan.Open();
            baglayici.Fill(tablo);
            baglan.Close();
            dataGridView1.DataSource = tablo;



        }

        public Form3 fmr3 { get; set; }

        private void button1_Click(object sender, EventArgs e)
        {
            frm3.ShowDialog();


        }



        public Form2 frm2 { get; set; }

        private void button2_Click(object sender, EventArgs e)
        {
            baglan.Open();
            string sqlsorgu = "Select * from kredi";
            DataTable tablo = new DataTable();
            MySqlDataAdapter baglayici = new MySqlDataAdapter();
            MySqlCommand komut = new MySqlCommand();
            komut.CommandText = sqlsorgu;
            komut.Connection = baglan;
            baglayici.SelectCommand = komut;
            baglayici.Fill(tablo);
            dataGridView1.DataSource = tablo;
            baglan.Close();
        }

        private void dataGridView1_CellContentClick_1(object sender, DataGridViewCellEventArgs e)
        {
            foreach (DataGridViewRow row in dataGridView1.Rows)
            {

                string RowType = row.Cells[1].Value.ToString();

                if (RowType == "toptanci")
                {

                    row.Cells[1].DefaultCellStyle.BackColor = System.Drawing.Color.Red;
                }

                else if (RowType == "perakendeci")
                {

                    row.Cells[1].DefaultCellStyle.BackColor = System.Drawing.Color.Red;

                }
            }
        }
          
        
        private void textBox1_TextChanged(object sender, EventArgs e)
        {
            if (textBox1.Text.Trim() == "")
            {
                DataTable tablo = new DataTable();
                tablo.Clear();
                MySqlDataAdapter baglayici = new MySqlDataAdapter("Select * from kredi", baglan);
                baglayici.Fill(tablo);
                dataGridView1.DataSource = tablo;
            }
            else
            {
                DataTable tablo = new DataTable();
                tablo.Clear();
                MySqlDataAdapter baglayici = new MySqlDataAdapter("Select * from kredi where musteriismi Like '%"+textBox1.Text+"%'", baglan);
                baglayici.Fill(tablo);
                dataGridView1.DataSource = tablo;

            }
        }

        private void button3_Click(object sender, EventArgs e)
        {

        }
    }
}

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