Here is additional information from RS on the website:
https://www.ricksteves.com/tours/italy/groups-visiting-the-vatican-colosseum
The Vatican Museums and Colosseum are world-renowned sights drawing huge crowds of visitors. Due to the overcrowding and unreliable availability for group reservations, our tours don't visit the inside of either sight as a group.
Colosseum
Our local guide walks the group around the outside of the Colosseum, explaining the significance and engineering of this ancient wonder. We also take an in-depth walking tour in the more enjoyable and less crowded Roman Forum, the true heart of ancient Rome.
Vatican Museums and Sistine Chapel
As a group, we visit Vatican City where a local expert will introduce us to the world’s smallest country, and the greatest church in Christendom: the awe-inspiring St. Peter's Basilica, home to Michelangelo's sublime Pietà.
If you would like to go inside the Colosseum and/or the Vatican Museums and Sistine Chapel, we encourage you to do so during your free time in Rome (or pre-/post-tour). Below you'll find our recommendations on when to visit and how to get reservations for each of the affected tours (this information is also available under Pre-tour Planning in your account.
Our free Rick Steves Audio Europe app also has great self-guided tours of both the Colosseum and Vatican Museums. Before you leave for your tour, we recommend downloading the app from the App Store (Apple phones) or Google Play (Android phones) and adding the Colosseum and Vatican Museums audio tours to your playlist.
Thank you for your understanding as we navigate overcrowding and group restrictions in the ever-changing tourism industry. We are always dedicated to bringing you the best experiences that Europe has to offer!
For example:
Best of Venice, Florence & Rome in 10 Days
Visiting the Vatican Museums/Sistine Chapel and Colosseum
We recommend booking tickets two months prior to the date you prefer to visit. Please note tickets may sell out quickly from the official site. Here are the best days and times to visit either or both sites based on your tour itinerary:
Day 8 at 11:00 a.m. or later for Vatican Museums
Day 9 at 1:30 p.m. or later for the Colosseum
Day 10 when the tour is over after breakfast
Post-tour if you're adding extra days
Please note: The Vatican Museums is closed most Sundays and may also close for special events and holidays. It's a good idea to check The Vatican Museums calendar before making reservations.