There was a long discussion of this in the early days of LactNet.  I
remember posting about my experience expressing colostrum/milk for Peter
when he was in NICU for three weeks.  All the moms who were expressing
stored their breast milk in a freezer in the NICU, by date, and you could
see the color very gradually fade from brilliant orange/yellow in the first
day or two, to only slightly yellow by three weeks postpartum.  There was
NO, repeat NO, abrupt shift from "colostrum" to "milk" at any time for any
of the moms (about 8 of us) who had our "milk" in the freezer.  I think
someone suggested in the earlier postings that prenatal vitamins which are
very high in beta carotene could be responsible for some of the brilliant
orange color.

Katherine A. Dettwyler, Ph.D.
Associate Professor of Anthropology and Nutrition
Texas A&M University