I had a bride call me today and she said that she went to a wedding where they had white marzipan details on a cake, and she wanted that too. I've never seen white marzipan and I told her that, but she was very insistent that she has seen white marzipan, so I told her I'd investigate. I looked it up in my pastry textbook and it said that you can color marzipan white by adding titanium dioxide to it, but then it should be used only for decorations that won't be eaten. Is there a way to color it with something edible? This has the potential to be a big PITA, I can feel it coming...
Amoretti in Los Angeles sells white marzipan. Though it is a bit expensive, you can pass the price along. http://www.amoretti.com
Also, Americolor has a white coloring. I think it is titanium dioxide, but it is as food safe as any other of their colors. Titanium dioxide is used in toothpaste and in many other edible things and in cosmetics.