If I take lunch to work I grill some chicken and cook up some pasta or cous cous the night before. Then I add loads of raw veg, like carrot, celery, peppers when its cold and take that with me.
Leftovers from the night before always work well too
Or if you have some time on your hands, homemade soup is always a good idea. There is a nice Ham Minestrone one on the website.
Or just have whatever you would have had on a red or green day, but just make sure that you have 1/3 superfree.