I've figured out how to make everything red as soon as the page is finished loading:

private void webBrowser1_LoadCompleted(object sender, NavigationEventArgs e)
{
    var doc = (IHTMLDocument2)webBrowser1.Document;
    foreach (IHTMLElement elem in doc.all)
    {
        elem.style.backgroundColor = "#ff0000";
    }
}

Now what if I want to make the element only change color when it's clicked? I see that elem has an onclick property, but it's type is dynamic so I don't know what to do with it. The documentation is pretty useless.