Easiest way I can think of is to give a #id to each main <li>
so something like:
background: #fff url(../../images/1/menu_button_orange.jpg) left top no-repeat;
background: #fff url(../../images/1/menu_button_pink.jpg) left top no-repeat;
giving each main list item a new id, then targeting each #id with its own colored background image. It’s quite a bit of CSS code, but should work just fine.
Hopefully you know what I mean, if not let me know and I can explain it in more detail.