I had to take a double take, lol
( I posted that pic a couple yrs ago when I ordered langshans from SandHill... someone says, "they ARE plums"....)
I really don't think they lay purple, any of them. And as far as I know the Langshans in the US came from Croads. Been on a couple good breeders websites of Lanshan, and they do not want you to even ask about purple eggs, must be they get it a lot.
I just googled that pic came up Croad Langshan Australia, as a famous myth of their "plum colored" or "purple egg"
"From the Croad Langshan Club Newsletter July 2013 (GB)
Egg colour madness
The club is frequently contacted by people wanting to buy a single hen ‘to lay purple eggs as I already have blue, brown and white’. I don’t know how to stop this – I see that some of the USA sites and forums have this nonsense about purple eggs, but no-one ever claims to have seen one! Probably this has become a distorted version of the ‘plum bloom’ on the Croad Langshan egg. This then gets reported as 'plum’, and then ‘purple’."
I think this sums the theory or myth of purple eggs, a 'plum bloom' any ole chicken egg can have it, the whitish dusting or film that washes off, just like on a plum. Also they mention European chickens at the time laid white eggs, so variety, brown etc. was uncommon I guess.
"Here is what Miss A.C. Croad, who originally developed the breed, has to say in her book “The Langshan fowl; its history and characteristics with some comments on its early opponents” p. 67 (1889).
"the Langshan seems to defy all set rules in this respect, and indulges in a charming variety; the tints are varied from the palest salmon to the darkest chestnut brown. On some there is a bloom like that on freshly-gathered fruit"