I get them all the time.
The chemicals that make the color get applied just before the cuticle as the egg passes through and leaves the shell gland. Sometimes the rate of travel changes.
It happens prior to the application of the bloom/cuticle. I get dark brown on one end and tan on the other often. A couple of times I had russet on the large end, lavender in the middle and plum on the small end. I'll post pics the next time I get one.