I'm from NZ and getting hold of white marshmallows is not an easy task.  I recently bought some jumbo marshmallows (made in the US) and after cutting them into small pieces the MMF turned out pretty well except that after one day it started to turn a creamy yellow colour.  Any ideas on how tio ensure the fondant is white would be appreciated

I make MMF and, if I want it to be really white, I use Wilton's "white-white" coloring in it. I just put a squirt or two in and knead it and it really whitens the stuff up! I've seen some people post on here that they don't like it because it changes the texture of their fondant, but I've used it many times with no problem. I've use it in butter cream too. I love the stuff.