no, you need to do something like:
$('input.sitebg').val('000000');
but you should really be using unique IDs if you can.
You can also get more specific, such as:
$('input[type=text].sitebg').val('000000');
EDIT:
do this to find your input based on the name attribute:
$('input[name=sitebg]').val('000000');