Hi, Dave. Milk paint is a casine based paint that has been around since the Egyptians were building the pyramids. For much more info, I suggest that you search for the Old Fashioned Milk Paint Companies site. They have a lot more info. There is also an article on using it in this month's issue of The Family Handyman.
By the way, some of the modern paint manufacturers advertise that they have milk paint colors. This is true but remember that they are the colors only and not true milk paint. Most of the colors will be those of natural pigments.