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()");