Coliseum to Vatican

It’s about 2.5 miles. If you are capable it’s an easy one hour walk.

Public bus (€2) with 1 change takes about 45 minutes. Metro (€4), also with 1 change, takes about a half hour.

Thank you both!

We did fine with public transport in Rome. We also used e-bikes renting from a place by the Circus Maximus, as well as lime bikes (we always rented e-bikes - Rome is NOT flat), and we used Uber. Uber in Rome also connects with the taxi system.

You will be on your feet for several hours for the tour - save your energy and take the Metro.

A single-ride BIT metro ticket is only € 1.50, and you can make the change at Termini from the B line, which stations nearer the Colosseum are on, to the A line, which stations near the Vatican are on, without a new ticket. Just follow the internal signs from one to the other and don't go outside the turnstiles. At termini, you'll want an A line train going in the direction of Battistini. It's not difficult, promise. :O)

https://www.atac.roma.it/en/tickets-and-passes

As we've rarely used public transit in Rome (More fun to walk!) we've sometimes just purchased a few BIT tickets at one time and tucked them away for longer hops if desired.

One important mention: if your trip is between 22 July - 9 September, the Ottaviano station near the Vatican will be closed for renovation work. You'd want to get off at the Cipro station.

https://www.wantedinrome.com/news/rome-closes-metro-a-early-jubilee-2025-works.html

A map of the metro system; ignore line C as you're unlikely to use it during your time in the city:
https://www.atac.roma.it/docs/default-source/mappe-tpl/mappa-metro-e-ferrovie-metropolitane.pdf?sfvrsn=e1e83890_20

Thank you all for your help!

A minor update to my post above:
The price of tickets and passes are reported to rise on July 1st, although there could be pushback. If it goes through, the current €1.50 price for a BIT ticket will increase to €2.00.

https://www.wantedinrome.com/news/rome-bus-subway-fares-set-to-rise.html

Just to add to comments on using public transport, the Tap&Go system seems to be fully operational so no need for tickets just use a contactless credit card (iphone, Google pay etc) on the readers, if you use same card or device each time daily, 3 day etc caps will automatically be applied. Last point - each person needs to use a different card/device.
Hope this helps.