On the excellent and convivial social network Mastodon, someone going by the handle “gay ornithopod” asked what turned out to be a fascinating question: My first response was that we can only say it’s not unusual for extant animals to change colour through ontogeny, so the null hypothesis would have to be that at least some sauropods (and other dinosaurs) did the same. But I don’t think we have any information on the specific coloration.