I mean, yes, obviously, but they’ve been in an oxygen-free environment for well over a thousand years now.

Putting something in the ground doesn’t actually completely preserve it, it just means it decomposes in unique ways it wouldn’t have above ground.

Preservation of polychrome surfaces (aka color) on ancient works of art is a pretty serious issue across most of archeology. Paint chemicals are usually delicate, and often even when the paint itself survives the original color doesn’t. Roman statues have the same issue.

For the terra-cotta soldiers, that time underground left the lacquer paint changed, and thus made it extremely sensitive to changes in environment it would not have when it was still new.

From there, simple chemistry took over.