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

Try following
 

<asp:TextBox ID="txtURL" runat="server" Text="&hello" TextMode="MultiLine" MaxLength="200" onFocus="this.style.background ='yellow'" onblur="this.style.background='white'"></asp:TextBox>

 

Thanks

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

<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">