Hi, maybe a silly question, but how on earth does white food colouring work to turn cream icing to white? It surely can't be bleaching it!
I can't explain how it works, but I've just discovered it and it's my new favorite thing! I use butter in my butter cream so it always comes out off-white. In goes the white coloring and presto! White as snow! I made some MMF last week and it, too, looked off white, so I kneaded in some white coloring and I was amazed at how white it got! I also had a small amount of pale pink butter cream left over and didn't want to waste it, so I added it to my new batch, put in a little white coloring to offset the pink, voila! White buttercream!
Sorry for rambling. I didn't even answer your question on how it works!!
It's liquid titanium dioxide-basically white edible paint.
I prefer not to use it in any quantity (I think that it changes the consistency of my BC), so to turn a yellowish/ivory BC white, I add a few DOTS of violet food color and get a nice, white BC. The violet cancels out the yellow.
My daughter who is a professional pastry chef said it is called "true white" or "white white" and can be extremely expensive. They can also be used in sugar art (edible and inedible).
Titanium dioxide is just about the purest white of the white pigments, (unlike white lead, which is a warm white, and zinc oxide, which is a cold white), and it's more-or-less biologically inert (unlike white lead, which is toxic as Hell, and zinc oxide, which can set off some people's allergies (I can't even handle galvanized iron for very long any more without starting to itch, and I'm told that zinc oxide diaper rash cream gave me diaper rash).
Most high-potency sunscreens contain titanium dioxide as a physical protectant, along with various chemical and biochemical protectants.
Unless it's an ingredient in, say, the blank media for the edible image I had made for the Printing Museum's cake, I've never used the stuff. I don't really expect BC to be pure white. At least, not the dense, non-whipped, cold-process BC I grew up on.