I'm a former conservation technician who used to work at a modest museum with a large collection of ancient/classical era sculptures, and was lucky enough to have a co-worker who did research on the polychrome techniques that were used on them.

From my understanding of his work, the reason modern recreations feel "plain" is because they are only using the pigments that they are able to find. It's possible that they statues were originally more intricately painted, but those larger flat surfaces are less likely to still hold specs of their original pigmentation, whereas the nooks and crannies of the more finely carved items (like saddles or fruit) are more likely to still retain a few flecks of pigment which can be analyzed. Ideally, conservators don't add in details that they don't know of/cannot prove what existed previously. (That's something restores do, but that's a different conversation). Which is why so many of them look rather plain. Hence why when they present those recreations, they usually brand it as a "recreation of what ____ possibly looked like"

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A modern example would be finding a few black threads and declaring that someone was wearing a black shirt (the plain-looking statue recreations, made with the evidence we have), vs. finding a few black threads and saying that someone was wearing a My Chemical Romance T-shirt (something that might have been true, but we have no way of proving it.)

Classical sculpture was anything but plain.

Sometimes, color was a function of material. A few of the most famous classical Greek statues were chryselephantine - i.e., armatures covered with thin sheets of ivory and gold. In these statues, the molded ivory (which was constantly moistened with water or olive oil to preserve its sheen) represented the subject's flesh, and was set off the flashing gold of the clothes and hair. The contrast was not life-like, and not intended to be; the object, rather, was to impress viewers with rich materials and shimmering surfaces. The famous statue of Athena in the Parthenon, and the even more famous Zeus at Olympia - both chryselephantine - astonished visitors through antiquity, and must have seemed to glow in the dimly-lit temples that housed them. The orator Dio Chrysostom, at least, could imagine no more spectacular depiction of a god than the Olympian Zeus.

Much later, the Romans experimented with sculptures of colored marble. Sometimes, the effect was intended to be realistic; to give one rather gruesome example, statues of the satyr Marsyas, whom Apollo skinned alive after defeating him in a musical contest, were sometimes fashioned from red-flecked Phrygian marble. Other examples were just intended to impress with their use of rare and expensive marble, like this Dacian originally from Trajan's Forum in Rome.

The most common sculptural media in both Classical Greece and Imperial Rome were bronze and white marble. Although bronze could not be painted, sculptors introduced alloys to give the metal more flesh-colored hues, and used inlays (most jarringly, glass eyes, like those of the famous Riace Bronzes) to add realism. The patina of bronze, however, was valued for its own sake. Roman collectors paid extravagant prices for statuettes fashioned from Corinthian Bronze, an alloy sometimes rumored to have been discovered after the Roman sack of Corinth, when melting bronze statues fused with other metals.

Marble statues were almost always painted (their current colorless appearance is a product of millennia of exposure, maladroit cleaning practices, and misconceived neoclassical conceptions of beauty). A few preserve traces of color, and more reveal at least the basic scheme of their original hues under ultraviolet light. Some, like the sculptures of the Parthenon, seem to have been downright garish by our standards (probably because the artists expected them to be viewed from forty feet below, and so took pains to make the details discernible). Most, however, seem to have been more or less naturalistic, like this statue of Venus from Beit Shean, which had reddish hair and a ruddy skin tone (women are typically paler than men in classical art, both from convention and in reflection of the fact that aristocratic ladies spent most of their time indoors). This Pompeian fresco shows two realistically colored paintings of goddesses, poised atop herms with natural skin and hair tones.

The colors of ancient sculpture, in short, ranged from the impressively stylized to the carefully naturalistic. The apparently muted nature of the pallets simply reflects the nature of the preserved remains.