I'm sure your question is about The Colosseum, but in regards to last usages of Roman amphitheaters I thought you might also be interested in the Roman amphitheatre in a French (formerly Roman) town called Arles. It was built around ten years after the Colosseum, and is currently in use as a popular bullfighting arena.

It wasn't in use continuously as an amphitheatre, however. For a thousand years in the middle, it had residences built in it, and wasn't restored to use until the early 19th century. The nearby city of Nîmes also contains a 19th-century-restored contemporary of the Colosseum, also still in use.

In terms of "last used", it depends on what type of use you mean. According to multiple sources (an online example ) an imperial decree in 404 ended gladiatorial combat, meaning that year was the official end of human combat and was more than likely a key moment in the history of he Colosseum (in terms of use). However, from what I've read animal hunts and entertainment continued on for a few decades, until money ran out to fund the importing of animals from Africa (around 600 AD is considered the end of all spectator sports).

I can't really find anything on the actual appearance of the structure, though it's worth noting that it was completely abandoned at some points and was ravaged by multiple earthquakes and stone-robbers. In the post-Gladiatorial combat and animal hunting era, the site was used for various religious orders and obviously continues to be used today - which is why your question of the "last use" of the Colosseum is a hard question to answer.

Hope this helps you!

I already submitted an addendum to the comment below about the Colosseum in Verona's use as an opera house, but as a scholar of the Catholic Church -- albeit more techincally a medievalist than a Romantic scholar -- I'd like to add a more direct response to your question (and I'm only a few centuries outside of my field in both directions).

Assuming you're referring to the Flavian Amphiteater (the famous "Colosseum" in Rome proper), it was rescued from an extensive period of neglect and marble-quarrying in the 18th century by Pope Benedict XIV, as part of the holy year he declared in his Benedictus Deus of 1750 -- sorry 'bout the Italian.

As a part of this event, he established stations of the cross, much like you'd find in a catholic church, around the Colosseum, as well as consecrating it in the memory of the martyrs of the early church in Rome. Here the sign (again, sorry for stock photo quality) that he erected in commemoration -- the cross he erected is still there, too.

As a consecrated space, it was then used as a part of the papal Via Crucis, attendance at whose stations were considered to grant indulgences. In his (frankly hilarious) 1846 "Pictures from Italy", Dickens mentions "There are several Crosses in Rome too, the kissing of which, confers indulgences for varying terms. That in the centre of the Coliseum [the cross erected by Benedict XIV], is worth a hundred days; and people may be seen kissing it from morning to night". While not an accurate statement of official Catholic policy, the passage certainly expresses the weight that the sacred space of the Colosseum still had for the people of Rome a hundred years after Benedict XIV -- in fact, in its capacity as a station in the Via Crucis, it is still used to his day.