With red earlobes you are looking at the blood underneath a thin clear membrane. With white earlobes there is a layer of white pigment so you can't see the red blood underneath. Supposedly with their black skin Silkie's earlobes can look blue, I've never had Silkies so I can't say for sure. All of that is genetic and is based on whether that skin is clear or has pigment. That much I know and really isn't based on circulation.
What I don't know is why a chicken's earlobes will change color like that. That does not look like frostbite to me and besides, if it were frostbite, it would be on the comb and wattles not the earlobe. I do not see any reason to think there is anything wrong.
If the person that selects which chickens get to breed pays attention to earlobe color you can get really bright sharp colors. If they don't pay attention to that you don't necessarily get real sharp colors. I don't know where you got your chickens but unless you are breeding for show or something else special, most people are not going to select a breeding bird because of earlobe color.
Now to Aart's circulation theory. When a hen is laying they tend to have bright red combs and wattles. If they are not laying those combs and wattles usually lose a lot of that bright color. I don't know the reason for that but it could have something to do with circulation. Do you know what causes that change
@aart @MercyChicken you said this is a recent change, not a permanent condition. Are they both laying right now or have they stopped for the winter? If they are not laying my guess is that the color change has something to do with that.
Other than that. I don't have any great thoughts about why they would change color. There have been other threads about this recently so I don't think it is unusual. I think it is a good question and shows you are observant. But I don't think it is anything to worry about.
