Why are my white egg laying chickens suddenly producing brown eggs

[benjamin163]

My mum has had chickens for 10 years.
Her latest batch have always laid white eggs.
Then suddenly the other day, brown ones appeared.
I have looked on this forum and seen it's not uncommon for brown eggs to turn white.
But I can't see any reports at all of white egg layers suddenly turning to brown ones.
Any ideas on why this has happened would be gratefully received.
(It's not a problem btw. The eggs are lovely. We're just interested enthusiasts)

 

[Pyxis]

Have you gotten any new chickens lately? And younger birds that may just now be coming into lay?

 

[benjamin163]

Thanks for the reply.
My mum got a four month old Sussex about six months ago.
She introduced it to the pen where there was already an unspecified grey which had been laying white eggs very successfully.
From the number of eggs my mum retrieves she presumes both of them have been laying (she was getting roughly a couple every three days) but suddenly a batch of three brown ones have appeared.
The only explanation we have is that these are the first eggs of the Sussex. But why would she have taken so long to start laying?
Your thoughts gratefully received.