Here is a way of changing back ground color for Textbox on validation failure. The key here is using Page_ClientValidate function in javascript.
<asp:TextBox ID=”TextBox1″ runat=”server”></asp:TextBox>

<asp:RequiredFieldValidator ID=”RequiredFieldValidator1″ runat=”server” ErrorMessage=”Please enter a value”
ControlToValidate=”TextBox1″></asp:RequiredFieldValidator>
<asp:Button ID=”Button1″ runat=”server” Text=”Button” OnClientClick=”btnClick();”
OnClick=”Button1_Click1″ />
<script type=”text/javascript”>
function btnClick()
{
if (!Page_ClientValidate())
{
document.getElementById(“TextBox1″).style.backgroundColor=”red”;
}
}

function ClearBackGround()
{
document.getElementById(“TextBox1″).style.backgroundColor=”";
}
</script>

In Code behind

TextBox1.Attributes.Add(“onkeypress”, “ClearBackGround()”);

By default, ASP.NET Web pages automatically validate that malicious users are not attempting to send script to your application. For more information, see Script Exploits Overview.