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The yellow color comes from excess bile in the stool. Bile is produced in the liver, stored in the gall bladder and excreted in the bowels. I would check to make sure none of the cats are jaundice. You can check the gums, roof of the mouth and the whites of the eyes. If they are yellow, then I would get bloodwork and look for a liver disease.
If none of the cats are jaundice, then I would guess that one of them had some excess bile in the body and that is their way of getting rid of it. In that case, I wouldn't be too concerned.
I hope this helps, let me know if you have any other questions.
You're very welcome.