Well isn't this interesting. Are you going to crack the egg to look at the colors of the interior?Nope it was laying on its side. Some of the UK breeders on FB said they see this once in a while, and just assume it is an uneven application of color during egg formation. The line is very striking though!
Last week I got an egg that had a nickel-sized oddity: the spot that was just off-set on one end was a slightly different color than the remainder of the shell and had a very thin layer of shell on top. The edges were very sharply defined where it met up with the rest of the shell. The only thing I could think of was that a piece of mucus or other material touched the shell as the egg was passing along and is spread out to cover the area and prevented the shell from adhering to underlying shell so there was a very thin weak spot.
Is the shell thicker or shinier where the darker color is present? If you wet the egg, does the color difference go away or become harder to see?
It almost looks like the colored part has a glossy finish and the 'normal' colored has a matte finish. How interesting.