custom validation css

hey consider this asp.net form textbox with validation <tr> <td class="tbl2col1" id="firstnameFirst">Your first name?</td> <td class="tbl2col2" id="firstnameSecond"><asp:TextBox ID="firstname" runat="server" /> <asp:RequiredFieldValidator ID="validateFirstname" runat="server" ControlToValidate="firstname" ErrorMessage="* You must enter your first name." Display="dynamic">* </asp:RequiredFieldValidator></td> </tr> When the validation fails, it will add the red * to the text. However, I want to change the class of element firstnameSecond to "tbl2col2 rederror". And then onchange when it gets rid of the *, i want the class of element firstnameSecond to go to "tbl2col2 greenerror". I can't figure out how I do this in AS ...