the first milk of a newly-lactating
breast.
it is perfectly designed by nature to clear a newborn baby's digestive tract of meconium, and can come in a varying range of colors, including yellowish and orangeish.
As lactogenesis progresses, colostrum becomes transitional milk and then finally mature milk.
The composition of colostrum is different for premature babies than for full-term babies, due to Nature's infinite wisdom.