namespace WebApplication3

{

    using System;

    using System.Web.UI;

    public partial class _Default : System.Web.UI.Page

    {

        protected void Page_Load(object sender, EventArgs e)

        {

            if (!Page.IsPostBack)

            {

                this.Renklendirelim();

            }

        }

        protected void Renklendirelim()

        {

            TextBox1.Attributes.Add("style", "border:solid 1px silver;");

            TextBox1.Attributes.Add("onfocus", "this.style.border='solid 1px red';");

            TextBox1.Attributes.Add("onblur", "this.style.border='solid 1px silver';");

        }

    }

}

<input name="TextBox1" type="text" id="TextBox1" onfocus="this.style.border='solid 1px red';" onblur="this.style.border='solid 1px silver';" style="border:solid 1px silver;" />