The fertilized egg will change from yellow to purple-black in 2-3 days, while the unfertilized egg will be yellow all its life, only when it begins to hatch naturally. The unfertilized eggs naturally hatch within 10- 14 days after delivery. I don't know why, but that's it this year.

The black ones in the picture are fertilized eggs [the earliest batch is about 30 days from today], and the light yellow ones have hatched [unfertilized, and it is about 14 days after birth today].