Yes, currently the Colosseum staff is requiring visitors to purchase tickets in advance and arrive with their ticket downloaded to their smartphone or tablet.
Advanced reservations are required if you would like to skip the ticket line at the Colosseum. The purchase of a guided tour will allow you to enter with a tour group and skip the ticket line. Please note that there is a mandatory security check for everyone entering the Colosseum and that this security line is different from the ticket line.