I would like to know if there is a way to use the
RequiredFieldValidator in such a way that would turn the label "Street
Address" red (see below) if the field were left blank and submitted.
Currently I only see the ability to add error text beside the field.
Is it possible to use the RequiredFieldValidator with an existing
label, and if so, how?

 Street Address   [     ]

You can add the required field validator to an existing text field in
the following way

<asp:textbox ID="Street" runat="server" TextMode="SingleLine"
TabIndex="1" CssClass="text"  /> <br> <asp:RequiredFieldValidator
id="StreetValidator" runat="server" ErrorMessage="Enter a street
address" ControlToValidate="Street" CssClass="text" 
Display="Dynamic"/>

You can change the css class to alter the colour of the error message.

Thanks, that is very helpful.

Is there a standard method for loading a new stylesheet on Postback in
ASP.NET? Or  a recommended way of changing the style for that one
particular CSS element?

Actually, sorry, this does not help, because the
RequiredFieldValidator only registers after a Postback. Where is the
ErrorMessage text on the initial page load and how do I get it to
display?