Thought I'd throw this out in case anyone is using old threads for their research.
For travelers wishing to self tour the arena (sans underground or upper tiers, which require escorted tours), there used to be two options on the coopculture website for pre-purchasing tickets. Option #1 allowed you to choose a scheduled entry time, and option #2 allowed entry anytime after 2:00 (14:00) PM. Option #1 was what visitors needed to go with if wanting or needing to visit earlier versus later in the day.
They've since deleted option #2 so now everyone pre-purchasing general-entry tickets must choose a specific entry time from whichever slots are available on a specific day, be that morning or afternoon. Link to the page here:
Note that they advise arrival at least 30 minutes before the scheduled time slot, and that, "tickets purchased can not be reimbursed or moved to another date or to another time slot on the same day."
Tourists with Roma Passes may still access the Colosseum anytime before last admittance hour (changes seasonally).
Editing to add: they've changed the website every whipstitch this past year, I swear. For most CURRENT info, always refer to the website versus old threads or guidebooks as it's very possible more changes could be in store!