Change textbox color if textbox is empty in asp.net using c#

Change textbox color if textbox is empty in asp.net using c#

 private void button1_Click(object sender, EventArgs e)  
 {  
 textBox1.BackColor = Color.White;  
 textBox2.BackColor = Color.White;  
 textBox3.BackColor = Color.White;  
 textBox4.BackColor = Color.White;  
 string[] text = new string[4];  
 text[0] = System.Convert.ToString(textBox1.Text);  
 text[1] = System.Convert.ToString(textBox2.Text);  
 text[2] = System.Convert.ToString(textBox3.Text);  
 text[3] = System.Convert.ToString(textBox4.Text);  
 for (int i = 0; i <= 3; i++)  
 {  
 if (text == "")  
 {  
 errorshow(i);  
 }  
 }  
 }  
 private void errorshow(int k)  
 {  
 switch (k)  
 {  
 case 0:  
 textBox1.BackColor = Color.Red;  
 break;  
 case 1:  
 textBox2.BackColor = Color.Red;  
 break;  
 case 2:  
 textBox3.BackColor = Color.Red;  
 break;  
 case 3:  
 textBox4.BackColor = Color.Red;  
 break;  
 }  
 }