I have one chicken (or I assume it's one) that has had the white part of the poop looking yellow for two days in a row, otherwise normal color and consistency. No chickens appear sick and I don't know which chicken it could be, I've just seen it under the roosts. No change in diet, they eat layer feed with the occasional leftovers. In the water I sometimes use some sort of vitamin thing for chickens, sometimes a little ACV and otherwise nothing. Any reason for concern? Since I don't know which one it is, it is possible that there are two chickens that have had this on different days, but this seems less likely. My immediate thought is that it could have been dehydrated, though they always have access to water, but this is only a wild guess.