We have white feathers turn up on birds after molt from time to time. Often after the next molt they go away. If I recall correctly we have had roosters get a few feathers like you picture, not quite as many. Note the feathers are not all white. Probably he has always had some with a bit of white at the base that is not noticeable and with this molt the distribution of white and color changed. We also had a mottled Java who lost almost all her white after a molt.
Here's an older BYC thread with others reporting this https://www.backyardchickens.com/threads/can-a-chicken-change-colors.249136/
Chickens can even change gender
- at least in external manifestations https://www.mypetchicken.com/backya...p/Can-a-hen-can-turn-into-a-rooster-H234.aspx https://animals.howstuffworks.com/birds/what-cluck-case-gender-changing-hen.htm
Here's an older BYC thread with others reporting this https://www.backyardchickens.com/threads/can-a-chicken-change-colors.249136/
Chickens can even change gender
