Changing the color of the text box when we are entering the details.and after that it should go to the previous color

Whatever styling name you have used to STYLE your text boxes...
You just duplicate the style then add :Hover to its name and modify the style

Lets say its called MyTextStyle

Then in your stylsheet it may appear as below...

The style below gives you white background and red text

.MyTextStyle
{
  Color: #FF0000;
  Background-color: #FFFFFF;
}

The style below gives you Black background and white text

.MyTextStyle:Hover
{
  Color: #FFFFFF;
  Background-color: #0000;
}

This is probably the BEST way to do it

Write this under TextChanged Event..

if(TextBox1.Text == string.Empty)
{
   TextBox1.Backcolor = Color.Red;
}
else
{
   TextBox1.Backcolor = Color.Green;
   TextBox1.Forecolor = Color.White;
}

<script type ="text/javascript">



    function ChangeBorder(obj, evt)

    {

        if(evt.type=="focus")

            obj.style.borderColor="red";

        else if(evt.type=="blur")

           obj.style.borderColor="black";

    }

   </script>

<asp:textbox id="txtFName" runat="server" onfocus="ChangeBorder(this,<br mode=" hold=" /><br mode=" onblur="ChangeBorder(this, event)" xmlns:asp="#unknown">