Did dinosaurs become more or less colourful through their lives?

On the excellent and convivial social network Mastodon, someone going by the handle “gay ornithopod” asked what turned out to be a fascinating question:

What are your thoughts on how the coloration of sauropods would change as they matured? What would you expect to see for example on this guy in comparison with an adult?

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.

At this point Adam Yates chipped in to observe that:

While we can’t know (as already discussed), it is my experience that the overwhelming pattern is for colours to become duller and patterns more muted as animals age.

That was surprising to me. I found myself thinking about all the birds that hatch out an undistinguished brown color, and develop spectacular colours as they age. Adam pointed out:

Yes there are those, but for everyone of those I’ll show you a lizard, snake or crocodylian with wonderful, vivid colours and patterns when young that fades with age (classic example is the Komodo Dragon).

I hadn’t know that Komodo Dragons hatch as colourful little critters, before later adopting their classic muted grey-green colour, but check out the photos and videos at ZooBorns:

Beautiful.

So this is interesting: it seems birds do one thing (become more colourful through ontogeny) while crocs and other reptiles do the opposite.

So the phylogenetic bracket is of little use to us here. Somewhere along the line from the most recent common ancestor of birds and crocs to modern birds, the ontogenetic trajectory flipped … but where along that line? With what implications for other dinosaur groups?

It’s a decent bet that primitive dinosaurs such as Saturnalia retained the ancestral condition, and became progressively less flamboyant through ontogeny, whereas bird-like raptors such as our old buddy Velociraptor assumed their most colourful plumage later in life. But what about sauropods? I’m not sure there’s any way to tell.

In classic palaeoart, sauropods were always a uniform greenish grey or brownish grey, or just plain grey. In more modern palaeoart we are seeing far more interesting colours and patterns: for example, the vivid black/white contrasts in John Conway’s Dreadnoughtus:

But if such patterning did occur, was it in juveniles or adults? (Or both, of course.)

I would like to understand why crocs and lizards have the trajectory they do. It’s easy to understand that juvenile birds are nondescript to avoid predation, but adults become more visible to attract mates. But how does the opposite trend make any sense? How is it of use to baby lizards to be highly visible?

Thoughts?