When you buy your skip-the-lines Colosseum ticket online, you will be emailed a PDF. This is not a ticket! It’s a voucher which is valid until December 31st of the year in which you bought it.
This voucher entitles you to a ticket to the Colosseum, Forum, and Palatine Hill, but it must still be exchanged for a ticket. You must print this voucher and bring it with you.
When you arrive at the Colosseum door or the Forum entrance (what’s the difference?) you will still need to convert your voucher into an official ticket that is then valid for the next three days.
At the Colosseum walk past the long line of people waiting to buy their tickets, and then enter the Colosseum through the right of the door. You may have to show your voucher to do this. Once inside you’ll see a few ticket windows on the right. At these you can hand over your voucher where it will be scanned and exchanged for your ticket/s
At the Forum/Palatine entrance you have to join the main line of people waiting to join tickets, but it will be very reduced in length as this is a much less popular entrance. At the ticket booth window they will exchange your voucher for ticket/s.
The tickets, which are credit-card sized, have a bar code on them which you scan at the turnstile/s, and then you’re in!