Bumping this back to top as it's an important little detail that's new for anyone using open-entry (not tours or scheduled-entry tickets) for the Colosseum/Forum/Palatine. From the website:
https://www.coopculture.it/en/colosseo-e-shop.cfm
"Open ticket for individuals:
The ticket colosseum/roman forum/palatine hill and current exhibitions is valid for 2 consecutive days, from the first use, for 1 entrance to the colosseum and 1 entrance to the roman forum/palatine hill. The ticket is valid until december 31, 2018. the open entrance tickets, issued starting from june 1st, 2018 can be used exclusively after 2 pm to access the colosseum.
This means that with a general-entry ("open") ticket purchased on or after June 1, you will not be able to access the Colosseum before 2:00 PM so plan accordingly. This does not seem to affect the Forum or Palatine.
So, if you want or need to get into the thing before that time of day, you'll want to buy a scheduled-time entry ticket or a tour. This also does not appear to affect Roma Pass holders - at least I'm not seeing anything yet which states that it does - but we'll see what develops, if anything.
Anyway, that bit of text might be easily missed amongst the oodles of other text on the site, or be a surprise to others using outdated threads for their info, thus the heads up! :O)
(Slightly edited since last posted)