It's just the shell gland laying down uneven layers of pigment around the shell, maybe due to differences in transit time through the shell gland/ oviduct. I have one hen that often lays "halvsies" eggs, half dark brown and half light brown. Like she just couldn't be bothered to finish the job. I doubt it's much reason for concern though.
That is cool! Maybe it was just a reproductive blip. Funky things can happen with those eggs. I usually don't worry about color variations if the hen is acting normally, but I have never had that much variation on a single egg.
I had a Golden Comet hen that used to sprinkle Hershey's cocoa on an egg occasionally. She laid a half dozen or so over a couple of months, till she ran out of cocoa. She was 16/18 months old. These pics were August 3 and October 11 respectively. You can see the egg in the nest on the left... GC
ETA; Her eggs returned to normal after.