how to change the color of mutiple words in textbox using asp.net and javascript

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ASP.NET

Javascript

how do i change the color of multiple words in textbox. i don't want to rich text box,is it possible to change it. I need without reading the string,the color changes only in textbox.
i try this,but the color is change not in textbox,i want to change the color in textbox.

window.pageLoad = function () {
                var textbox = document.getElementById('<%= TextBox1.ClientID %>');
                var span = document.getElementById('txt');
                var text = textbox.value;
                var mytool_array = text.split(":");
               alert(text);
                if (mytool_array.length > 0) {
                 span.innerHTML = mytool_array[0] +""  + mytool_array[1] + "";         
                } }
            .mOver
 {
   color:Aqua; 
 }
<asp:TextBox ID="TextBox11" text="Asp:net" onblur="this.style.color='brown'" CssClass="disableCss" runat="server"  TextMode="MultiLine"><span style="color:Olive" id="txt"></span>