Apologies if this has already been posted but I haven't seen it.
"A Night at the Colosseum" is now available on the official website's ticketing site.
https://ticketing.colosseo.it/en/categorie/visite-guidate/
It's a 60-minute guided tour of the first ring, the arena and the underground. I expect they'll only offer these until late in fall or end of the year - times for the latest slots in the evening might change- but with Jubilee coming in 2025, it's possible they'll extend them through the winter. Guess we'll see!
To add: They do not mention if you're allowed to linger anywhere in the structure after the conclusion of the tour but I'm guessing not. For sure that won't be allowed in the underground as it isn't allowed for any other tours which include that area.