Javascript Run-time Error When Using A Server Certificate

I have recently set up a server certificate on a web site.
Under certain conditions I need to change the color of a html span
element.

I do this using the following javascript function called from the
onreset attribute of the form element.

function removeWarningMsg()
{
if (isIE) {
document.all.Warning.style.color = "<%=BACKGROUND_COLOUR%>";
}
return true;
}

This was working perfectly fine until I applied the server certificate
to the website. Now when executing this line of code the following error occurs:
Error: 'document.all.warning.style' is null or not an object.