The Colosseum entrance requires you to arrive 15 minutes early, so the timing is quite strict; don't arrive too early. I recommend buying a ticket for the top floor pavilion, which allows you to take the elevator directly up for panoramic views and also see the Roman Forum from above. It's much less crowded, so you don't have to squeeze through the throngs.
The only downside is that the time slots are unpredictable. For example, we were assigned tickets for 10:50, and by the time we got to the Roman Forum, there was a nearly 50-meter-long queue. The sun was blazing with no shade, and there was only one entrance to the entire square, so we decided to give up and go back to the hotel.