Ok, open your quick reply template (whatever it is called) and you should have something that looks like this:
Quote:
<input type="checkbox" checked>
The bold part is what you need to include for it to be checked, by default. That doesn't need to be in quote marks or anything just type out: checked. Also make sure it is the input box for your signature, not something else.
You should be able to but it would require some more extensive hacking. You may want to search VBulletin.org for some more hacks for changing colors of users.