Sudoku.Form1.textBox4_TextChanged C# (CSharp) Method

        private void textBox4_TextChanged(object sender, EventArgs e)
        {
            if (textBox4.Text != "")
            {
                if (textBox4.Text == textBox1.Text ||
                    textBox4.Text == textBox2.Text ||
                    textBox4.Text == textBox7.Text ||
                    textBox4.Text == textBox8.Text ||
                    textBox4.Text == textBox3.Text ||
                    textBox4.Text == textBox14.Text ||
                    textBox4.Text == textBox16.Text)
                {
                    textBox4.BackColor = Color.Red;
                }
                else
                {
                    textBox4.BackColor = Color.White;
                }
            }
            else
            {
                textBox4.BackColor = Color.White;
            }
            congratulations();
        }