Guinea fowl white face turned yellow

[Sydney65]

Someone asked this years ago then didn't follow up. Picture from that bird :
A friend has this with 2 older goons now. Has anyone seen or dealt with this? The most I can find is that a toxin in onions destroys RBCs and can cause this, but seems like you'd have very sickly birds at that point.

 

[Eggcessive]

Could your friend post a picture of their birds? I know nothing of guinea fowl, but I had 2 ancona hens with white earlobes who would turn yellow in the fall when they were molting and not laying. They would become white again in spring.