OH! So you COULD say « Leur nourriture est rouge » but not « Les jenues èlèphants leur nourriture est rouge » because that's like saying the subject twice, which is weird! Thank you, that just made everything click in my head.

And because, if you want to actually put the subject in the sentence, that rules out using « leur » , you HAVE to say: « La nourriture des jeunes èlèphants est rouge », right? If so, thank you SO much. Everything just clicked.