what can be carried into colosseum?

As far as I know there hasn't been any "new" limits on what you can bring in since 2015. They tried, very, very briefly back then, to restrict backpacks and bags of nebulous sizes and that didn't work very well. So, the rule has been no LARGE (camping-size) backpacks, bulky bags or suitcases/trolleys. I believe there may be a ban on selfie sticks and tripods as well - although it isn't spelled out on the coopculture website - but even if allowed, those items are so roundly banned at other attractions that I wouldn't bring them anyway.

"For security reasons all visitors and their luggage shall be screened. To facilitate the security checks please insert any object (including mobile phone) in the bag/backpack or in the tray to be included in the X-ray. It is confirmed the prohibition of bringing backpacks , camping , bulky bags and luggage / trolley, can be introduced medium and small size backpacks to shoulder, which must be checked, as any other bag, with metal detectors, opened and visually inspected by the Urbe Security Institute, responsible for the entrance inspection."

Your cane (necessary for mobility/stability challenges, I assume) and water bottle will be fine. It might be worth mention that if your cane is for balance/stability, I wouldn't recommend reserving a Belvedere/Upper Tiers tour, if you were thinking about it? That particular section is not advised for visitors with mobility limitations. Have fun!! :O)

thank you. just standard tour.