Elderly hens changing colour

[KirbyG]

Hey everyone! I have 2 elderly hens, a brown shaver (Teriyaki) and a giant pure white girl (Biggun). They have just gone through/are in the middle of their winter moult and both have gained a couple of black or partially black feathers. Does anyone have any info on what could have caused this?
Biggun has a half speckled black tail feather, some tiny black speckling on random body feathers and a tiny black neck feather.
Teriyaki has 2 half black speckled tail feathers.
At first I thought the tail feathers were just dirty but they've definitely grown back a new colour.

 

[BlindLemonChicken]

I don’t know what causes it but I have seen the same with my chickens. My speckled sussex were especially obvious because they got whiter with each molt

 

[TwoCrows]

I too have seen this in some of my senior birds. White feathering in dark colored birds is common, barring on feathers too. I think over time color genes get corrupted or possibly genes from their distant relatives starts to show through.

 

[FarmerGirl101]

That is normal. I haven't had senior birds in a while but even my younger ones get some colour changes in molts. One of my has a white patch now and my roosters also started getting a lot more white in their tails.